Solar and heat pumps: understand first, then decide

Who this is for

You own a house in Germany. You want solar panels (PV), maybe a battery, and a heat pump to replace an old oil or gas boiler. Maybe you want to charge an electric car too.

Installers will call you, send you offers and explain things in their own way. Each offer looks different. Prices are hard to compare. This book helps you understand the parts, ask the right questions and compare offers fairly.

It is written for people who are not experts. Some readers do not speak English or German as a first language, so the sentences are short and every technical word is explained when it first appears. If you still get stuck, the glossary in Part 7 is there for you.

How the book is built

The book follows the order of a real project. Each part builds on the one before it.

You can read it from top to bottom. You can also use the list on the left (or the Contents button on a phone) to jump to a chapter. If you are new to the topic, read Parts 1 to 3 in order. They give you the words you need for the rest.

How each chapter works

• A short box at the top tells you what you will learn.
• Many chapters have a table with three columns: good, acceptable and red flag. You can hold an offer next to the table and check each line.
• Boxes marked Ask give you questions to send to an installer.
• A Key points box at the end sums up the chapter.

The one rule

Compare offers on the same scope. One installer includes a new meter cabinet, scaffolding and surge protection. Another leaves them out and looks cheaper. The cheaper one is often not cheaper at all. Most of Part 5 is about this single problem.

A few honest limits

• This is not advice for your house. It helps you ask better questions. A site visit by a qualified installer or energy expert is still needed.
• Funding and prices change. Where we give numbers, we give a date. Check the current rules before you act.
• The book is vendor-neutral. No company pays to be in it. When we name brands, we do it to show how the technology differs, not to sell anything.
• Local control is a priority here. Many chapters explain how to keep control of your own system without a vendor cloud. If you do not care about this, you can skip those parts and the rest still works.

Where the content comes from

The book grew out of a real renovation project: a house from the 1970s, an oil boiler, and more than twenty offers for solar and heat pumps. The author built a large comparison workbook to make sense of them. This book teaches everything in that workbook, in a logical order. Names and prices of the real offers are left out.

Four parts, one idea

A modern energy-ready home has up to four big parts.

1. Solar panels (PV). They make electricity from sunlight. Most of it comes in summer and around noon.
2. A battery. It stores solar power for the evening and night.
3. A heat pump. It heats the house and the hot water, and it uses electricity to do it.
4. An electric car (optional). It is a big battery on wheels and a big electricity user.

Each part is useful on its own. They work much better together. The idea that connects them is simple: make your own power, and use it yourself, at the right time.

Power from your own roof costs you a few cents per kWh over the life of the system. Power from the grid costs much more. Power you send to the grid earns you only a small feed-in payment. So the best use of a solar kWh is to use it in your own house.

How the parts help each other

• The sun shines at noon. The house uses little power at noon. The battery moves that power to the evening.
• A heat pump needs power all day. In summer, it can make hot water from your solar surplus. A hot water tank is a cheap "battery" for heat.
• A car can charge from the roof on a sunny weekend.
• At night and in winter, you buy power from the grid. A dynamic tariff can make that power cheaper if the system buys when prices are low.

To make this happen, the parts must be able to talk to each other. That is why Part 4 of this book is about control and open interfaces. You can build a good system with parts that do not talk to each other. It will just be less smart.

What can go wrong

Most problems in these projects do not come from bad technology. They come from:

• a heat pump that is too big, or radiators that need hot water the pump cannot make efficiently
• offers that leave out important items, so the total price looks low
• a system you cannot control or fix without the installer's cloud
• funding applications done in the wrong order, so the grant is lost

Each of these is avoidable. Each one has a chapter in this book.

The five steps of a project

You will notice that "buy" comes late. Good projects spend more time on steps 1 to 4 than on the purchase.

An example house

Throughout the book we come back to one example, to make numbers real. It is a house from 1972 with about 200 m² of heated area, four people, radiators (no underfloor heating) and an oil boiler of about 25 years. It uses about 4,000 kWh of electricity a year, plus about 4,000 litres of oil. The owner wants PV, a battery, a heat pump and later an electric car.

Your house will be different. The reasoning is the same.

Power is speed, energy is distance

Think of a car. Power is like speed: how fast something is happening right now. Energy is like distance: how much has happened in total.

• kW (kilowatt) is power. A heat pump that needs 2 kW is using energy at a rate of 2 kW right now.
• kWh (kilowatt-hour) is energy. If that heat pump runs for 3 hours at 2 kW, it used 6 kWh.

Your electricity bill is in kWh. Your battery is measured in kWh. The size of a heat pump is given in kW.

A second picture: a water tap. kW is how wide the tap is open. kWh is how much water is in the bucket.

What kWp means

kWp means "kilowatt peak". It is the power of a solar array in perfect lab conditions. These are called STC (Standard Test Conditions): a fixed strong light, and the cell at 25 °C.

On a real roof you almost never get those conditions. The sun is lower, there are clouds, and the cells get hot. So a 10 kWp array usually delivers less than 10 kW. It is a size label, not a promise.

Over a full year, a good roof in southern Germany produces about 1,000 to 1,100 kWh per kWp. An example from near Munich: 1,060 kWh per kWp per year is a healthy planning value. A 15 kWp system then makes about 15,900 kWh a year.

STC and NOCT

You will see two more short words on datasheets.

• STC is the lab condition described above.
• NOCT (Nominal Operating Cell Temperature) is closer to real life: a warmer cell, less light. NOCT numbers tell you more about what the panel does on your roof.

Do not worry about the details now. Just remember that the big number on the label is a lab number.

Euros per kWp

The most useful way to compare PV offers is price per kWp. You divide the total price by the size of the array in kWp.

Example: a system with 15 kWp for 24,000 € is 1,600 € per kWp.

For southern Germany in 2026, a complete system with battery and wallbox often falls between 1,500 and 1,800 € per kWp. Much lower or much higher than that deserves a closer look at what is in the offer. Chapter 26 shows how to do the comparison properly, because a low price per kWp can hide missing items.

A few numbers for your own house

These are rough numbers for Germany. Your house will differ.

Add up the first three and you get roughly 8,500 to 11,500 kWh. That tells you why a larger array makes sense once a heat pump and a car arrive.

The chain in four steps

1. Modules turn sunlight into direct current (DC). There are no moving parts. Many modules are connected in a row. This row is called a string.
2. Strings go to the inverter. A roof often has two or more strings, for example one per roof side.
3. The inverter changes DC into alternating current (AC), the kind your house and the grid use. It also does MPPT: it keeps finding the best working point of the modules. Light and temperature change all day, so this point keeps moving. A hybrid inverter also runs the battery.
4. House and grid. The house uses the power first. Extra power goes into the battery, then to the grid.

Between these parts you also find cables, isolators, surge protection and a meter. Chapter 8 covers them.

What decides the yearly yield

People often study panel types for hours. In practice, the roof matters more.

Direction. A south roof gives the most energy over a year. An east-west roof gives a lower peak at noon, but more power in the morning and evening. That can be useful if you use power then. A south-east roof gives less in the late afternoon. Plan for that.

Tilt. In Germany, about 30 to 35 degrees is ideal for yearly yield. A steeper roof gives more in winter.

Shade. Shade costs more than its size suggests. If one cell is in shadow, the whole string can lose power. A chimney, a tree or a dormer can matter. Two things help: an inverter with several MPP trackers, and power optimisers at module level. Use optimisers only if you have real shade. Modern panels handle light shade quite well.

Temperature. Hot cells make less power. On a summer roof, cells can reach 60 to 70 °C. Chapter 5 shows how to read this in the datasheet.

Dirt and snow. Rain cleans most panels well enough in Germany. A steep roof sheds snow faster.

Where the battery and heat pump connect

With a hybrid inverter, the battery connects to the same device on the DC side. This is efficient and neat. With an AC-coupled battery, a second inverter does the work. Chapter 6 explains the difference.

The heat pump connects to the house grid like any large appliance. It becomes smart when something tells it to run while the sun shines. Part 4 shows how.

What to ask about yield numbers

Offers often include a yearly yield estimate in kWh. Ask:

A number far above 1,100 kWh per kWp in southern Germany needs a good reason.

Why modules deserve a careful look

The module is the part of a PV system that fails least. It also sets the upper limit of your yield and the lifetime of the system. The number printed on the front, the Wp value, is the least useful number for comparing modules. Temperature behaviour, degradation and the warranty tell you much more over 25 years.

The numbers, one by one

Nominal power (Wp). This is the power at STC (the lab condition from chapter 3). Two modules with the same Wp can deliver different energy on a real roof. Use Wp to size your array, not to judge quality. Common today: 430 to 480 Wp.

Efficiency (%). This is how much of the light becomes electricity per square metre. It matters when your roof area is the limit. On a large roof, a cheaper module with lower efficiency can be the better deal.

Cell technology. PERC is the older standard. TOPCon (n-type) and HJT (heterojunction) are the current generation. They lose less power in heat, degrade less in the first year and work better in weak light. HJT has the best temperature behaviour but costs more. Avoid polycrystalline modules or offers that do not name the technology.

Temperature coefficient (%/°C). This is the most under-rated number. It tells you how much power the module loses for every degree above 25 °C. On a summer roof, cells reach 60 to 70 °C. At 65 °C, a module with -0.34 %/°C loses about 13.6 %. A module with -0.29 %/°C loses about 11.6 %. So a module with a lower label Wp but a better coefficient can produce more on a hot day.

Bifacial. A bifacial module also collects light on its back. This only pays on a flat roof, a ground mount or a raised frame with a bright surface below. On a normal tiled pitched roof the gain is small. Do not pay extra for it there.

Glass-glass or glass-foil. Glass-glass modules are stronger, handle hail better and often come with longer warranties. They are a bit heavier.

The two warranties

Many people mix these up. A "25-year warranty" can mean two very different things.

• Product warranty covers physical defects: frame, junction box, glass, delamination. Good makers give 15 to 25 years.
• Performance warranty covers the power output over time. Current n-type modules promise about 1 % loss in year 1 and about 0.4 % per year after that. This gives about 87 to 90 % of the original power after 25 to 30 years.

A 25-year performance warranty with a 10-year product warranty is a mismatch. Ask for both numbers, in years.

Tier-1 does not mean best

"Tier-1" is a list by Bloomberg of manufacturers that banks trust. It is a ranking of bankability, not of quality. It still matters a little: a warranty is only worth something if the company still exists in year 20. Check that the warranty is handled by a German or EU company, not only by a head office far away.

Strength and safety

• Mechanical load. Standard is 5,400 Pa (snow, pushing) and 2,400 Pa (wind, pulling). Bavaria has real snow loads. Check that the module covers your zone.
• Hail. Look for a hail test or class.
• Certificates. IEC 61215 (design) and IEC 61730 (safety) are the minimum. In Germany also check the fire class of the module for your roof.

Check list

What the inverter does

The inverter turns the DC from your modules into AC for the house. It tracks the best working point (MPPT). In a hybrid system it also charges and discharges the battery and can run the house during a power cut.

It is also the part most likely to fail. Plan for one replacement during the life of the system, typically after 10 to 15 years. So the warranty length and the service matter.

Four types

For a new system with a battery (now or later), a hybrid inverter is the usual choice.

AC-coupled or DC-coupled battery

• DC-coupled (hybrid): the battery charges straight from the panels. There is one conversion step. This is about 2 to 3 % more efficient. It can also store power that the inverter would otherwise have to cut off.
• AC-coupled: the battery has its own inverter and connects on the AC side. It is flexible for adding a battery to an old system, but it has an extra conversion loss.

For a new build, DC-coupled is the better choice. Be careful if the offer does not say which one it is.

Efficiency

Two numbers exist: the peak efficiency and the European efficiency. Peak efficiency is a marketing number. The European (or CEC) efficiency is a weighted average across realistic light levels. It is the honest one. If an offer only quotes the peak value, ask for the European value. Good is 97.5 % or more.

Size ratio and clipping

Most arrays are a bit bigger than the inverter. The panels rarely reach their full label power, so a slightly smaller inverter loses almost nothing and runs at a better average efficiency.

The ratio is DC power (kWp) divided by inverter AC power (kW).

• 1.1 to 1.25 is the comfortable range.
• Above about 1.3, the inverter cuts the top of the curve on sunny days. This is called clipping. You lose a little yield.
• 1.0 or lower means you pay for an inverter you never fully use.

Example: a 14.4 kWp array with a 15 kW inverter is a ratio of 0.96. Not harmful, but you may be paying for more inverter than you need. A 10 to 12 kW hybrid would clip almost nothing.

The other direction is also a trap. An 8 kW inverter on a 15.6 kWp array is a ratio of almost 2. It will clip a lot. Some inverters also have a maximum allowed array size. One real offer had a 10 kW inverter that the datasheet limited to 15 kWp, and the planned array was 17.9 kWp. The offer then needed a second inverter for extra money. Always check the datasheet limit.

MPP trackers

Each MPP tracker looks after one string, on its own. If your roof has an east side and a west side, you need at least two trackers. One tracker across two directions drags the better side down to the level of the weaker one. If you have shade, more trackers help more.

Ask how many MPP trackers the inverter has and how the strings are split across the roof sides.

Heat and placement

All inverters reduce their output when it gets too hot, often above 40 to 45 °C around them. This is called derating. A hot cellar with no air or a wall in full sun can cost you yield at noon in summer.

• Put the inverter in a cool, shaded, ventilated place with free space around it.
• Check the protection class. IP65 or higher is fine for outdoors or a garage.
• Fan-cooled units and units with a large heat sink handle heat well. Cheap units derate earlier.

Backup, monitoring and warranty

• Backup. See chapter 18. "Backup-ready" often means no backup hardware is included.
• Monitoring and interfaces. You want per-string data and a local interface such as Modbus TCP. Cloud-only monitoring is a risk: a lock-in and a problem if the vendor shuts it down. Chapters 20 to 22 go deeper.
• Warranty. 5 to 10 years is standard. Ten years with an option to extend is good. Fewer than 5 years is a red flag. Watch the conditions: one brand gives only 2 years by default and extends to 5 years if you register in its cloud portal. That is a hidden link to the cloud.

Check list

What a battery is for

In Germany today, a battery rarely pays for itself through price trading alone. Its value comes from three things:

1. More self-consumption. It moves solar power from noon to the evening.
2. Backup power in a blackout, if the system is set up for it (chapter 18).
3. Smart use of tariffs. With a dynamic tariff the battery can charge when power is cheap (chapter 24).

Decide which of these matters to you. It changes the size you need.

Usable, not nominal

Sellers love to quote the nominal capacity. What you can really use is the usable capacity, which is the nominal capacity times the depth of discharge (DoD).

Example: a 10 kWh nominal LFP battery with 95 % DoD gives 9.5 kWh. An older type with 80 % DoD gives 8 kWh. Always compare the price per usable kWh.

Chemistry

• LFP (lithium iron phosphate) is now the standard for home storage. It has no cobalt, is much safer in heat, and lasts 6,000 to 10,000 cycles. It is a little bulkier, which does not matter in a house.
• NMC is more compact but runs hotter and ages faster.
• Lead-acid is old technology. Avoid it.

For a house, LFP is almost always the right answer.

Depth of discharge and state of health

• DoD is how much of the battery you may use. LFP allows 90 to 100 %.
• SoH (state of health) is the remaining capacity compared to new. A warranty promises a minimum SoH at the end, for example 70 %.

Cycles and warranty

A cycle is one full charge and discharge. At one cycle per day, 6,000 cycles last about 16 years. Look for 6,000 cycles or more down to 70 or 80 % capacity.

Battery warranties usually have two limits: years and energy throughput. For example: "10 years or 20 MWh, whichever comes first, with at least 70 % capacity left." Which limit hits first for your use? A "10-year warranty" with a low throughput limit can end after 6 years. Ask for years, throughput and the minimum SoH.

Round-trip efficiency

This is energy out divided by energy in, including the inverter. Every lost percent is power you paid to store and did not get back. DC-coupled systems do best. 90 % or more is good. Below 85 % is weak.

Modular or fixed

A modular battery lets you add capacity later, for example when you buy a car or the heat pump load grows. Ask for the maximum number of modules, the price of an extension, and whether modules of different ages can be mixed. A fixed battery forces you to buy the final size now.

A sensible plan is to start with about 10 kWh and keep room to grow to 15 kWh.

Temperature and place

Battery cells like 15 to 30 °C. An unheated garage in a Bavarian winter can block charging or shorten life if the battery is not rated for it. A hot room ages cells faster. Put the battery in a stable, cool, dry room. Check the rated temperature range in the datasheet against the place where it will sit.

Safety certificates

Look for VDE-AR-E 2510-50 (the German home storage standard), IEC 62619, and a built-in battery management system (BMS) with cell balancing. The chemistry is the first safety layer. Certificates and the BMS are the second.

Check list

Why the boring parts matter

A premium module on a rusting rail, or an undersized cable, is a bad deal. The parts around the modules decide whether the system is safe and reliable for 25 years. They are also the first place an installer saves money to make an offer look cheaper. Since you cannot see them once the system is built, check them in the offer.

Mounting

Racking and roof hooks. Aluminium rails with stainless steel (A2 or A4) screws last 25 years or more. The roof hooks must fit your roof type (tile, slate, trapezoid sheet, flat roof) and be sealed correctly. Every hole in the roof is a possible leak. Mixed metals cause corrosion.

Replacement tiles. On a tiled roof, the hooks sit under the tiles. The tile above them is lifted by the hook and can crack. Metal replacement tiles at each hook position avoid this. Some tile makers refuse their own warranty without them. In real offers this item is often "not included" or "optional". It can cost over 1,000 € for a larger roof, so ask.

Load calculation. Snow and wind loads are different at each location. Ask for a project-specific load calculation, or for the flat-roof ballast plan, not a generic layout.

Electrical parts

DC and AC cables. The cross-section must be big enough to keep the voltage drop on the DC side under about 1 %. Use UV-resistant solar cable outdoors. Too-small cables cause a silent yield loss and heat.

Isolators, surge protection (SPD) and RCD. These protect against fire and lightning surges. They are often left out of cheap offers.

• SPD means surge protective device. Look for Type 1 plus 2 on the AC side and Type 2 on the DC side. A thunderstorm nearby can send a surge through the cables.
• Many inverters have an arc detector (AFCI). That is good, but it is not the same as surge protection.
• The RCD must be the right type for the inverter.

Earthing and equipotential bonding. The metal parts of the system must be connected to the earth. Ask whether a ground rod or foundation earth, and the cable to the meter cabinet, are included.

Meter cabinet. An old cabinet (for example from 1972) often does not meet today's rules (VDE-AR-N 4100). The grid operator may refuse to start the system if the cabinet is not updated. Renewing the meter cabinet can cost from about 1,300 € to over 4,000 €, depending on whether you have one or two cabinets and what must be done. Many offers exclude it, sometimes with a small warning. Ask in writing who does it and who pays.

Main fuse (SLS). The selective main line circuit breaker protects the house connection. Its size limits how much power you can take and give at once. A new heat pump, a wallbox and battery charging together may need more than an old connection offers. Ask the installer to check the connection and the SLS (chapter 19).

Smart meter and energy meter. To optimise self-consumption, the system needs a meter at the grid connection point. For dynamic tariffs you need a smart meter system (iMSys). Check what is included and where it measures.

Energy management and interfaces. This ties PV, battery, heat pump and car together (Part 4). Open protocols (Modbus, EEBus, SG-Ready) avoid lock-in.

Scaffolding

Roof work needs scaffolding. Some offers include it. Some say "if needed" and charge 250 € or more. Some leave it to you. If you also plan roof or facade work, share the scaffolding between the jobs.

Wallbox and EV readiness

A wallbox has two parts: the device and the connection (cable, circuit breaker, RCD, network cable). Many offers include the connection but not the device, which costs about 1,000 to 1,500 € more. If you plan a car, provision the conduit and the electrical capacity now. Retrofitting is more expensive. An 11 kW wallbox with solar surplus charging is a good target.

Heat-pump related parts

These belong to the heat pump offer (chapters 12 to 14): buffer tank, hydraulic diagram, hot water tank, check of the radiators, and hydraulic balancing. We list them here so you see the full picture of "balance of system".

Check list

Step 1: Add up your future use

Start with what you use now, then add what is coming.

Use your own bills for the first line. For the heat pump, wait for the heat load calculation in chapter 13.

Step 2: Choose the PV size

Southern Germany gives about 1,000 to 1,100 kWh per kWp per year. To cover roughly the full yearly use you would need about 9 to 10 kWp. But solar power is made in summer, while a heat pump needs power in winter. So many homes with a heat pump choose a larger array, often 14 to 16 kWp, to get more power in the dark months and in spring and autumn.

Panels are cheap compared to everything else in the system. Scaffolding, inverter, meter cabinet and labour are the same for a small or a larger array. So going from 10 to 15 kWp often adds much less than 50 % to the price. Always ask for the price of the extra modules.

Check the roof first. How many modules fit? A roof that can hold 17 kWp is wasted if the offer fills only 10 kWp. Several offers in the real project offered 9 to 10 kWp on a roof that could hold about 17.6 kWp. Ask each installer for the maximum sensible layout.

If you may add modules later, think about the cost: another scaffold, more electrical work, a new grid registration, and perhaps a second feed-in tariff block. Ask the installer to price both: fill now, or expand later. The answer is often "fill now".

Step 3: Choose the battery size

A rule of thumb for a house with 4,000 to 10,000 kWh use: 10 to 15 kWh usable. Larger is not always better. A 25 kWh battery for a house that uses 4,000 kWh may never fill up. In the real project, one offer came with 25.6 kWh. It was far too large for the stated goal of 10 to 12 kWh.

Prefer a modular battery. Start at about 10 kWh and add later if the car or heat pump load grows.

Also check the power of the battery (kW), not only the size (kWh). A battery with 5 kW discharge power cannot run a heat pump, an oven and a car charger at once in a blackout.

Step 4: Check the inverter and the rest

• Inverter: 12 to 15 kW AC for a 17 kWp array fits the DC to AC ratio of chapter 6.
• Heat pump: sized by the heat load, not by the PV (chapter 13).
• Connection: check that the house connection and main fuse can take the combined peak load (chapter 19).

Typical prices

For southern Germany in 2026, complete PV systems with battery and some extras fell mostly between 1,500 and 1,800 € per kWp. The cheapest honest systems in the real project were around 1,300 to 1,400 € per kWp. The most expensive offers reached 2,000 to 2,500 € per kWp. Prices change. Use the range only to spot offers that are far outside it, and ask why.

Please also remember: PV and batteries on a private house have 0 % VAT in Germany at the time of writing. Heat pumps carry 19 %. Always check whether an offer is net or gross.

A worked example

A house uses 4,000 kWh now. A heat pump will add 3,500 kWh, an electric car 2,500 kWh. Total about 10,000 kWh. The roof can hold 41 modules of 430 W, or about 17.6 kWp. Expected yield is 17.6 × 1,060 = about 18,700 kWh per year. A battery of 12 to 15 kWh usable fits. An inverter of 15 kW AC gives a ratio of 1.17.

A fridge in reverse

A fridge moves heat from inside the box to the room behind it. That is why the back of a fridge feels warm. A heat pump does the same thing, but on purpose and on a big scale. It takes heat from the outside air (or from the ground or water), and moves it into your heating system.

It moves heat. It does not make heat by burning something. Moving heat takes much less energy than making it.

The four steps

Inside the heat pump a special liquid, the refrigerant, goes round in a loop.

1. Evaporator. The cold refrigerant takes heat from the outside air and turns into a gas. Even air at 0 °C holds a lot of heat for this.
2. Compressor. It squeezes the gas. Squeezing makes it hot. This step needs electricity.
3. Condenser. The hot gas gives its heat to the water of your heating system and turns back into a liquid.
4. Expansion valve. The pressure drops. The liquid gets cold again, and the loop starts over.

How much heat you get

For every 1 kWh of electricity, a heat pump delivers about 3 to 5 kWh of heat. An oil boiler gives you less than 1 kWh of heat for 1 kWh of oil. A direct electric heater gives exactly 1 kWh. That is why a heat pump is so attractive.

The ratio of heat out to electricity in is called the COP (coefficient of performance). A COP of 4 means 4 kWh of heat per 1 kWh of electricity.

What limits the COP

The key rule: the bigger the temperature gap the pump must bridge, the lower its COP. This gap is called the lift. It is the difference between the source (outside air) and the water you want (the flow temperature to your radiators).

There is even a simple formula for the best possible value, with temperatures in Kelvin (°C plus 273):

COP maximum = hot side / (hot side minus cold side)

Example: outside air at 0 °C is 273 K. A flow temperature of 35 °C is 308 K. The maximum is 308 / 35, about 8.8. For a flow temperature of 55 °C (328 K) the maximum is 328 / 55, about 6.0. Real machines reach about half of these ideal numbers, but the trend is the same: a lower flow temperature gives a much better COP.

Every 5 °C you can take off the flow temperature gives a noticeable improvement. This is why flow temperature has its own chapter (chapter 12), and why old radiators are the main worry in a renovation.

Heat source types

• Air-to-water is the most common. It is cheaper to install. Its efficiency falls on very cold days.
• Ground source (with probes or collectors) uses the stable ground temperature. It is more efficient but costs more and needs drilling or space.
• Water source uses groundwater. It needs a permit and good water.

Most houses use air-to-water, and so does the rest of this book.

What a heat pump needs from your house

• A heating system that works at a moderate water temperature.
• A correct size for the heat your house needs (chapter 13).
• A place outside with enough space for air and low noise for the neighbours (chapter 15).

Three numbers

All three say how much heat you get per unit of electricity. They differ in when and where they are measured.

The flattering number

Vendors love to quote the COP at A7/W35. This means outside air at 7 °C and a flow temperature of 35 °C. That is a mild day and a low flow temperature. It is the easiest condition for the machine. It makes the unit look great.

A cold day matters more, because that is when you need the most heat. So also ask for the COP at A-7/W35 (outside air at minus 7 °C), and the COP at the flow temperature you really need.

SCOP35 and SCOP55

SCOP is given for two flow temperatures:

• SCOP35 is for low flow temperature (35 °C), typical for underfloor heating.
• SCOP55 is for 55 °C, typical for old radiators.

If your house has old radiators, SCOP55 is the number to look at, not SCOP35. SCOP55 is always lower. Some offers only show the 35 °C number because it looks better.

What JAZ is worth

JAZ is measured over a real year with real weather, hot water, defrost cycles and your radiators. It is the honest number. A JAZ of 3 or higher is a rough threshold for sensible running cost, and some subsidy rules link to it. 4 or higher is excellent.

You cannot know your JAZ before the machine runs. But a good installer can calculate an expected JAZ for your design flow temperature. Ask for it.

Why the real number is lower than the brochure

• Defrost cycles in cold, damp weather take energy.
• Hot water at 55 to 60 °C needs more lift than heating.
• Short cycling (a machine too big for the house) wastes energy.
• A high flow temperature, forced by small radiators, lowers the COP.
• Electric backup heaters add direct electricity.

You influence most of these. The next chapters show how.

Check list

What it is

The flow temperature (Vorlauftemperatur) is the temperature of the water that leaves the heat pump and goes to your radiators or underfloor heating. A heat pump works best when this is low. As chapter 10 showed, every degree of lift costs efficiency.

A rough rule: going from 50 °C to 45 °C flow saves about 8 to 12 % of the electricity.

Two different questions

People mix up two questions.

1. Can the heat pump reach the temperature my radiators need? This is about the maximum flow temperature of the machine. Modern R290 (propane) monobloc heat pumps reach 70 to 75 °C. So in principle they can work with old radiators. Always check the current datasheet of the exact model. Older documents sometimes list lower values.
2. What temperature will I actually run? This is about cost. You want the lowest temperature that keeps the house warm. A machine that can do 75 °C should not run there.

Why old houses are the issue

Radiators of the 1970s and 1980s were often designed for 70 °C or more, with an old boiler. A heat pump at 70 °C has a poor COP. In a renovated house with better insulation and new windows, the same radiators can heat the house with water of 45 to 50 °C. That is because the house needs much less heat, so the radiators do not need to be as hot.

So flow temperature and insulation belong together. Chapter 13 shows how to find the heat your house really needs.

How to bring the temperature down

• Insulate and replace windows. Less heat is needed, so lower water temperature is enough.
• Larger or better radiators, or surface heating (floor, wall, ceiling). More area gives the same heat with cooler water.
• Hydraulic balancing. See below.
• Fan convectors (radiators with a fan) for key rooms, if needed.

A good target for a retrofit is a design flow temperature of 45 °C or less. In the check list below, 55 °C or more on small existing radiators is a red flag.

Hydraulic balancing

In an unbalanced system, the radiators near the pump get too much water and the far ones too little. The usual reaction is to raise the temperature. Hydraulic balancing adjusts the flow to each radiator so that the system works at the lowest possible temperature.

The best method is Verfahren B. It is a room-by-room calculation and setting. It is required for the BEG subsidy (chapter 32) and it pays back in lower running cost. Make sure it is in the offer, not an afterthought. How such a calculation works is described, for example, by the engineering office Cedroswan: hydraulic balancing with Verfahren B (in German).

The single-pipe problem

In a single-pipe system (Einrohrheizung), the radiators sit in one long loop, one after another. The water cools down as it passes each one. This design needs high flow temperatures and is the worst case for a heat pump.

One offer in a real project noted that the house had a single-pipe system. The advice: check every offer for it, and consider converting to a two-pipe system or upgrading the radiators during the renovation, before or together with the heat pump. A full conversion is not always needed: the article single-pipe heating and heat pumps (in German) shows what a hydraulic analysis can do for such systems.

The emitter check

A good installer does a room-by-room check of whether your existing radiators can give the needed heat at the design flow temperature. Many offers skip this and quote only a kW number. That is the root of disappointing results after a retrofit.

Check list

The heat load

The heat load (Heizlast) is the heating power your house needs on the coldest design day of the year. It is calculated, in Germany, according to DIN EN 12831. This number, in kW, should decide the size of the heat pump.

An offer without a heat load calculation is guessing. And when installers guess, they usually guess big.

Do not size on the old boiler

Many people think: "My oil boiler has 25 kW, so I need a big heat pump." That is wrong. Old boilers are almost always far too big. Also, if you insulate and change windows, the need drops further.

A quick way to see your yearly heat need: 1 litre of heating oil holds about 10 kWh. A house using 4,000 litres per year needs about 40,000 kWh of heat. In a real project, one calculation showed 39,240 kWh before the renovation and 11,983 kWh after it (insulation of facade and roof plus triple-glazed windows). That is less than a third.

A renovated house of about 200 m² often needs 8 to 12 kW at the design day. Real offers in the project came with 16 kW and even 22 kW. Both are too big.

Why too big is bad

A too-big heat pump meets the small heat demand very quickly, then switches off. After a short time it starts again. This is short cycling.

• Efficiency (JAZ) drops.
• The compressor wears out faster.
• The noise events increase, because starting is when the machine is loudest.

A correctly sized machine that modulates (adjusts its power up and down) runs for long, quiet, efficient periods. It is the opposite of an on/off machine.

Why too small is also not good

If the machine is too small, an electric backup heater has to help on cold days. That heater uses direct electricity and ruins the yearly efficiency. A good size lets the heat pump cover nearly all cold days without the heater. Many designs size to cover the design day, with the heater as a safety reserve only.

What a good heat load calculation contains

• A room-by-room list with areas and U-values of walls, roof, windows and floor.
• The assumptions for air changes and the design outside temperature.
• The result for the whole building, in kW.
• A hot water allowance if the heat pump also makes hot water.

It is often done by an energy consultant (Energieberater), who may also write the funding application. Some installers include it in the offer. If the energy consultant is part of the offer, that is a strong point. For an idea of what a technical review of an existing house covers, see Cedroswan's page heat pumps in existing buildings (in German).

Link to the radiators

The kW figure alone is not enough. The radiators must be able to give that heat at a low flow temperature (chapter 12). The two checks belong together.

Modulation

A modulating heat pump (inverter-driven compressor) changes its output. A range of 25 to 100 % means it can run as low as a quarter of its full power. This fits a house with a low part-load for most of the winter. Single-speed (on/off) machines are a red flag.

Check list

Refrigerant

The refrigerant is the working fluid in the loop (chapter 10). Its type matters for efficiency, for the environment and for how long the rules will allow it.

*GWP is the global warming potential. Lower is better.

R290 is the future-proof choice. It also earns an efficiency bonus in some funding rules (chapter 32).

Monobloc or split

• Monobloc: all refrigerant stays in one outdoor unit. Only water pipes go into the house. There is no refrigerant work inside the house, it is simple, and it suits R290 well.
• Split: an outdoor and an indoor unit, joined by refrigerant lines. This needs certified refrigerant handling. It can help where there is little space or long pipe runs.

For most houses with R290, a monobloc is the simpler choice.

Modulation

A wide modulation range (for example 25 to 100 %) lets the machine run long and gently (chapter 13). Single-speed units cycle on and off and are less efficient and louder.

Defrost and cold-weather behaviour

At about 0 °C with high humidity, the outdoor unit ices up. It has to defrost by reversing the cycle for a short time, which costs energy. Good units start defrost on demand, based on a sensor, not on a simple timer. Check the rated heat output and COP at A-7/W35. That is the cold-day number, and a cold day is when you need the heat most.

Hot water and legionella

If the heat pump also makes hot water, three points matter.

• Tank size and reheat time. The tank should fit the family and have enough coil area.
• Legionella protection. Water in the tank is heated to about 60 °C from time to time to kill bacteria. If the compressor can reach 60 °C on its own, efficiency stays good. If an electric immersion heater does it often, the efficiency suffers badly. Check which is the case.
• Circulation. If your hot water circulates in a loop through the house (as in many older houses), the energy loss is large. Ask how the design handles it.

Buffer tank and hydraulics

A correctly sized buffer tank keeps the heat pump from cycling too often and gives it a place to put its heat (chapter 16). A hydraulic diagram in the offer shows how all parts connect. Over- and under-sizing of the buffer and the hot water tank both reduce efficiency. If no diagram is in the offer, ask for one.

Warranty and service

• Compressor and whole-unit warranty can be different. Ask for both in years.
• A heat pump should last 15 to 20 years. A local service partner matters.
• Check whether a maintenance contract is needed to keep the warranty.

What a complete heat pump offer contains

Compare these items across offers (see also chapter 26):

• Exact model, output in kW, refrigerant, monobloc or split.
• Heat load calculation and the design flow temperature.
• SCOP and a calculated JAZ, plus the COP at A-7/W35.
• Noise data and the planned location (chapter 15).
• Hot water tank size, buffer size, hydraulic diagram.
• Radiator check and hydraulic balancing (Verfahren B).
• Removal of the old boiler and the oil tank (it is often excluded and costs extra).
• Electrical work and the new circuit.
• The funding application: who does it, and is the energy consultant included?
• Payment plan, warranty, maintenance, service partner.

Check list

Two numbers that look alike

Noise data come in two forms. They are easy to mix up.

• Sound power level (Lw) is how loud the machine is at the source. It does not depend on distance.
• Sound pressure level (Lp) is how loud it is at a given distance, for example at 3 m, or at your neighbour's window.

Both are in dB(A). The sound pressure at the neighbour's window is what the law cares about. It depends on the sound power, the distance, and what is in between (walls, hedges, open space).

A machine of 55 dB(A) sound power can be about 33 dB(A) at 5 m in open space, and quieter again further away. A wall nearby that reflects the sound makes it louder.

The German rule: TA Lärm

The TA Lärm limits the noise at the neighbour's nearest window. For example, in a purely residential area the limit at night is around 35 dB(A). In other area types the limits are higher. Night means 22:00 to 06:00. Your local building office can tell you which area type applies.

On a tight plot, for example in Munich, the noise rules can decide where the unit may stand, and even which unit you can choose.

What to ask for

• The sound power Lw of the unit.
• The sound pressure at a stated distance.
• The night mode figure. Many units can run slower and quieter at night.
• A statement that the planned location meets TA Lärm at the neighbour's window.

Some installers offer a noise calculation. It is worth having when the plot is small.

Placement tips

• Keep distance from the property boundary and from bedroom windows, yours and your neighbour's.
• Avoid corners and narrow gaps between walls, which reflect and increase the sound.
• Leave free space for air in front and behind the unit. Check the datasheet.
• Think about snow, falling ice and condensate from the defrost. The condensate must drain away and must not freeze on a path.
• Keep the pipes between the unit and the house short. Long pipes lose heat.
• With R290, siting rules apply, for example for distances to openings and cellar windows. Follow the installer's and the maker's instructions.

Check list

An oil boiler does not care

An oil burner makes hot water on demand. It does not matter if the flow is 50 or 70 °C, if few or many radiators are open, or if it switches on and off often. Its efficiency hardly changes. You can steer it bluntly.

A heat pump is different. It has two hard rules.

Rule 1: run long and gentle, not hot and short

A heat pump is most efficient when it runs for long periods at the lowest flow temperature that keeps the house warm (chapter 12). Going from 50 °C to 45 °C saves about 8 to 12 % of the electricity. So the goal is not "set 50 °C and forget it". The goal is to run as cool as comfort allows. You do that with the heating curve.

Rule 2: never choke the flow

When the compressor runs, the heat pump needs a minimum water flow to carry the heat away. If you close most radiator valves, the heat has nowhere to go. The machine then cycles on and off quickly, and this wears the compressor.

An oil-boiler habit is to close valves and throttle the circulation to save energy. With a heat pump that is risky. A buffer tank in the system gives a safety margin, but it does not make throttling a good idea.

The heating curve

The heating curve (Heizkurve) tells the heat pump what flow temperature to make at each outdoor temperature. Cold outside means higher flow. Mild outside means lower flow. An outdoor sensor measures the temperature, and the machine adjusts itself all the time.

What you do by hand with an oil boiler in spring and autumn, turn it down when it is mild, the curve does automatically, and more smoothly.

How to tune it:

1. Start low. Choose a low curve at commissioning.
2. Wait for a cold day. Raise the curve in small steps only if some rooms stay too cool.
3. Find the lowest curve that keeps you comfortable. Every degree lower saves money.

Controllers show this as a base point and an end point, or as a graph with a comfort factor. If the system still needs high temperatures, a heating optimisation for existing buildings (in German) by a heating engineer can help.

The room setpoint

On a weather-compensated heat pump, changing the room target temperature in the app moves the whole heating curve up or down in parallel. It is not a thermostat measuring your room. Higher setpoint means higher flow means higher cost. It is a handy fine adjustment, but it has a price.

Modes

• Program or automatic: follows a schedule with comfort and eco phases. This is the recommended default.
• Constant comfort or eco: holds one level, for example eco while you are away.
• Standby or hot water only (summer): heating off, frost protection only.

A small night setback of 2 to 3 °C is fine. A deep setback does not pay: the morning reheat costs more than the night saves.

Radiator valves

You can keep thermostatic valves, but the rule changes. Leave the radiators in the reference rooms fully open. They secure the minimum flow. Use the valves only to trim a room that is too warm. Do not shut down large parts of the system.

If you want less heat, lower the curve or the setpoint. Do not lower the flow.

Do and do not

Hot water in summer

In summer the heat pump only makes hot water. Do it in the sunny hours if you can (chapter 24). A small boost on solar surplus stores heat for free.

If your heat pump has an open interface

If it has an interface such as Modbus (chapter 21), you can set the safe levers, mode and setpoint, from a home automation system. Many makers protect the raw "maximum flow" setting, but mode and setpoint give nearly the same effect.

Why radiators barely cool

Many heat pumps can run in reverse and cool the house. Whether that helps depends on the heating surfaces.

Radiators barely cool. They have too little surface to cool a room much. Worse, a cold radiator surface can fall below the dew point. Then water condenses on it and drips. That can cause moisture damage.

What works

Effective cooling needs a lot of surface area.

• Underfloor or wall heating gives gentle cooling.
• Fan coils (Gebläsekonvektoren) have their own fan and cool actively. In a radiator house, they are the realistic retrofit.

Dew point protection

Safe cooling needs a dew point or humidity sensor that stops the cooling before water condenses. Check that it is part of the offer.

What fan coils cost

A fan coil is more than the device. You also need pipes to each unit and a condensate drain, because each unit produces water. The price depends strongly on the routing. Ask for a price per room, including pipes and condensate.

The funding question

A common worry: "If I switch on cooling, I lose the funding." The usual position under the BEG: the heat pump is funded as a heating replacement. The cooling function is allowed as an extra, as long as the unit mainly serves as heating and meets the efficiency rules. Cooling as the main purpose is funded only in non-residential buildings. Some installers say "never activate it". That is over-careful and not the actual rule.

The rules change, so confirm the details with the energy consultant who files your application. They carry the responsibility for funding compliance.

A safe path: install the heat pump now as a funded heating system. Add the cooling parts later at your own cost, and keep heating as the main use.

Two words, two levels

German offers use two terms. They sound alike but mean different levels.

• Ersatzstrom is a simple backup, often switched manually. It is often single-phase and covers only selected sockets or circuits. There can be a short interruption.
• Notstrom (or USV, a UPS) is automatic and almost seamless. Ideally it is 3-phase and covers the whole house. Your devices keep running without noticing the outage.

Three questions

1. Seamless or with a gap? A switch-over time below about 20 ms means computers and routers do not notice. Some systems take several seconds. A few start slowly, up to a minute or more. After a gap, devices restart. This matters if you work from home or have sensitive equipment.

2. Whole house or selected circuits? "Selected circuits" means only the lines chosen in advance are backed up. Everything else is dark. Ask exactly which circuits.

3. How many amps per phase? This decides which loads can run together. A system limited to 35 A per phase can not do what one with 63 A can.

The hidden cost

Backup is almost always a separate item: a backup box or switch-over device plus wiring in the meter cabinet. "Backup-ready" often means the system could do it. The hardware is not in the price. Many offers describe a battery as "backup-capable" and then leave out the box. In a blackout, you get nothing.

Also note: some offers say backup is "not possible in cascade", meaning not with several inverters together. Ask what the alternative is.

Why 3-phase matters for a heat pump

A heat pump and a big wallbox use three phases. If your backup is only single-phase, lights, fridge and router keep working, but the heat pump will not run. For whole-house backup you need a 3-phase backup-capable hybrid inverter, a backup box where needed, and correct wiring.

Also check:

• Unbalanced loads. With 3-phase backup you want the battery to supply each phase as needed. Some inverters can give full power on one phase (100 % unbalanced), others cannot.
• Peak power. A heat pump draws a big starting current. Check that the battery can deliver it. Keep the total peak load in backup mode inside the system limit (for example 10 kW).
• Black start. Can the system restart itself in a blackout when the battery is empty and the sun is shining? Some can ("black-start capable").

Differences between brands

Brands differ much here. In one comparison for the real project:

• Some systems have built-in 3-phase backup with a switch-over around 10 to 20 ms and no extra box (up to about 10 kW).
• Some need a separate backup box, and switch in the range of seconds.
• Some start slowly (20 to 90 seconds) and warn you first.

The exact numbers belong to specific models and change. Use them as examples of the range and ask for the figure for the model in your offer.

Wiring risk

Backup needs correct changeover wiring. If it is wired badly, an inverter fault can drop the house. Ask who designs and checks the wiring, and whether a protective device (MCCB) is included.

Check list

As of 2026. Rules and limits change. Ask your grid operator if in doubt.

A few rules always come up with PV and heat pumps. They sound strict. For a household that mostly uses its own power, they are usually harmless.

Smart meter and the 60 % limit

Above a certain system size (typically above 7 kWp) you need an intelligent metering system (iMSys, a smart meter). It is installed by the metering point operator (Messstellenbetreiber), not by your PV company. The wait can be several weeks.

Until a smart meter with a control box is active, a temporary feed-in limit of 60 % of the system power often applies (Solarspitzengesetz). Two points ease the worry:

• It limits only what you send to the grid. It does not limit what you use yourself.
• It is lifted once the smart meter gateway and the control box are in operation.

For a system built for self-consumption, with battery, heat pump and car, this matters little.

§14a: controllable devices

Heat pumps and wallboxes above 4.2 kW count as controllable consumption devices. Under §14a EnWG you get reduced grid fees. In return, the grid operator may briefly reduce their power in peak hours. This needs a meter cabinet set up for it. In practice the reduction is rare and mild, while the saving runs all year.

Some offers prepare this in the meter cabinet (a free field and wiring). Others say that the control box and the wiring to the heat pump "may be at your cost". Ask clearly, as chapter 29 suggests.

Battery charging from the grid above 4.2 kW can also fall under §14a.

The house connection and the main fuse

A house built in 1972 may have a small connection. A heat pump, a wallbox with 11 kW, and the battery charging from the grid in cheap hours may all run at the same time. Together they can ask for more than the connection gives.

Ask your installer to check:

• the size of the main fuse (SLS) (for example 35, 50 or 63 A),
• whether the house connection is large enough,
• whether the energy manager should limit the total load.

In the real project, the grid operator said that at PV registration they check the connection and may raise it to 63 A, at a cost of 0 to 350 €. That answered an open question for all installers. Ask your own operator.

What you must do

You do not need to do much:

• Make sure your installer handles the grid registration and the coordination with the metering operator. Most do.
• Know that modern inverters are ready for remote reduction of power.
• Register your system in the Marktstammdatenregister (MaStR), within about a month of commissioning. You do this yourself. It is free and online. Chapter 36 covers it.

The positive side

The controllability these rules ask for is the same controllability you want yourself. It is the base for dynamic tariffs and for self-consumption optimisation. Compliance and your own plan use the same technology.

Two ways to control a system

Cloud control. You use the maker's app. The app talks to the maker's servers, and the servers talk to your inverter or heat pump. Everything goes through the internet and through the company.

Local control. Your own devices talk to the inverter or heat pump directly, on your home network. The internet is not needed. You can use the maker's app as well, but you do not depend on it.

Both work on a good day. The difference shows when something changes.

Four risks of cloud-only

1. Changes. The maker can change the app, remove a feature or end support for old models.
2. Subscriptions. A feature that is free today can move behind a payment. One example: a maker offers automatic dynamic-tariff charging only in a paid cloud plan of about 36 € per year. It is not a large sum. It shows the principle, because a function you thought you bought can become a rental.
3. Warranty tied to the cloud. Some makers give a longer warranty only if you register your system in their portal. That makes the portal a requirement.
4. Outages and shutdown. If the servers go down, or the company disappears, a cloud-only system loses its smart functions. If your internet fails, so does your control.

There is also a point about privacy: your consumption data goes to a company you do not know well.

What you gain with local control

• Your own rules. Charge the battery when power is cheap. Send solar surplus to the heat pump or the car. No subscription.
• Independence. It keeps working if the cloud goes away.
• Integration. One system, for example Home Assistant or evcc, can control the inverter, battery, heat pump and wallbox together, even from different makers.
• Longer life for the system. An inverter or heat pump stays useful after its cloud stops.

Do you need it?

Not everyone does. If you want a simple system that "just works" and you are happy with the maker's app, a cloud system is fine. But if any of these is true, local control should be a selection criterion:

• You want to use a dynamic tariff with your own logic.
• You want to combine PV, battery, heat pump and car from different makers.
• You do not want to depend on a subscription or on the company staying in business for 20 years.
• You like to tinker with a home automation system.

A note about honesty

Local control is not a free lunch. You, or a helper, must set it up. You become responsible for rules that run the battery. If your controller fails, the system must fall back to a safe mode (chapter 24). A good local setup is also a hobby. If that does not appeal to you, pick a system with a good maker app, and make sure the basic functions work without the cloud.

Make it part of your decision

Treat the control question as you treat the warranty. It is easy to check before you buy and hard to fix afterwards. The next chapters give you the vocabulary (chapter 21) and the brand picture (chapters 22 and 23). In Part 5 the control check becomes a line in your offer comparison.

Reading and writing

Two different things hide behind "local control".

• Reading means you see data: PV power, battery level, grid flow. Almost every device allows this.
• Writing means you give commands: "charge the battery now", "set this limit". This is where brands differ a lot.

When you check a system, always ask separately: can I read locally? Can I write locally?

Modbus

Modbus is an old, simple, open protocol used in industry. A device on your home network (a small computer, or Home Assistant) asks the inverter for values or sets values. Modbus TCP runs over the network cable. Modbus RTU runs over a serial line (RS485).

If an inverter has documented local Modbus TCP with read and write, you can do nearly everything: read PV, battery and grid data, set charge and discharge power, limit export. It is the gold standard for home automation.

Points to check:

• Some makers must enable Modbus at commissioning. The installer may need an account to do it.
• Some need the right dongle or network stick. A stick that looks right may not support Modbus.
• A wired network connection (LAN) is more reliable than Wi-Fi. Plan a cable to the inverter, the heat pump and the wallbox while the walls are open.
• Using Modbus must not void the warranty. Ask in writing.

SG-Ready

SG-Ready is a very simple signal for heat pumps. It is two contacts (terminals) that read the state of two switches. Four combinations give four modes.

The heat pump only reads the contacts. Something else must close them:

• your PV inverter when it sees surplus,
• a home automation system (with a relay),
• the grid operator (§14a).

SG-Ready is universal and cheap, but it is coarse. It cannot say "make 43 °C" or "charge the tank to 52 °C". What "boost" means is decided by the heat pump's own controller. Also check who wires the contacts. The job often falls between the heating and the electrical trade.

EEBus

EEBus is a newer European standard for talking between appliances (heat pump, wallbox) and an energy manager. It is more detailed than SG-Ready. It is the modern successor in the long run.

Do not expect too much today:

• Many EEBus links give power limits and measurements, not set points such as temperatures.
• The open reference code cannot be used as is in Home Assistant.
• It is great when both devices speak it and a manager controls them (for example from the same family of products). It is not the same as being controllable from your own home automation.

Other local interfaces

• REST / HTTP API: a documented web interface on the device. Good for reading, sometimes for writing.
• MQTT: a light message system many hobby tools use. Some devices or add-on boxes offer it.
• Proprietary local APIs: e.g. a manufacturer's local interface that only works with its own partner software. Open to partners, not to you.

Tools you will hear about

Home Assistant is open-source software for home automation. It runs on a small computer in your home. It has ready-made integrations for many inverters and heat pumps, some from the community (called HACS integrations). It can read and control devices locally.

evcc is open-source software focused on solar-aware charging of electric cars and battery control. It knows many inverters, wallboxes and meters. It handles the logic of self-consumption, minimum charge and tariffs, so you do not program it all yourself.

Both can run on the same small computer.

Open or closed? A short table

Check list

Brand behaviour changes with firmware and new models. The notes below come from manufacturer documents and owner and installer reports, collected in 2026. They are direction, not lab results. Check the exact model before you buy.

The read and write trap

When you start sending commands, you become the battery manager. The inverter does what you say and may stop its own self-consumption logic. You then need to take care of three things: sensible self-consumption, a minimum charge level, and a failsafe: what happens if your controller crashes? A good setup returns to the inverter's own behaviour.

This is why evcc is a good choice. It takes the manager role and handles these points cleanly.

Brands differ in how they allow reading and writing together:

• Some inverters allow a third-party controller to read and write over Modbus at the same time, with a clean fallback to self-consumption when idle.
• One well-known brand has a limit in its manual: while "inverter control via Modbus" is on, sending and receiving at the same time is not possible. The workaround is to read through its web API and write through Modbus, on separate channels.
• Some limit the number of Modbus connections at once, or lock settings behind an installer account.
• Some use a watchdog: a setpoint expires after, for example, 60 seconds, so your controller must resend it every 30 seconds. That is a safety feature, but you must plan for it.

Tiers for local control

This list simplifies. It is built on how well a platform allows reading and writing over a local interface, how mature the Home Assistant support is, and how closed the cloud model is.

Notes by platform

These notes give you the character of each platform. They are examples, not endorsements.

Sungrow (hybrid inverter with its own battery). The most installed hybrid in many markets. Local Modbus TCP with read and write is documented and the manual allows third-party energy managers. Best-documented Home Assistant integration. Built-in 3-phase backup of about 20 ms with no extra box, up to roughly 10 kW. Occasional battery communication or firmware faults are reported. Check backup wiring.

SMA (Sunny Tripower with Home Storage). German, with a very high reliability record and the best efficiency scores in independent tests. Native local Modbus and good Home Assistant support. Backup works with a gap of about 5 to 7 seconds and needs a backup unit. Often the highest price per kWp. The SMA Home Manager 2.0 is a local energy manager and works with dynamic tariffs.

Huawei (SUN2000 with LUNA2000). Class-leading efficiency, local Modbus TCP, strong Home Assistant support. Backup needs a separate backup box. The older LUNA S0 generation is less attractive than S1. Some buyers consider brand and geopolitical risk for a 20-year asset.

Sigenergy (SigenStor). Modern, modular, true 3-phase backup with a gateway, native Modbus TCP, an integrated DC EV charger for vehicle-to-home. The installer must enable Modbus at commissioning. A young company with no decade-long record, and one voluntary connector replacement programme in 2025.

Fronius (GEN24 / Verto with BYD battery). Excellent reliability, active fan cooling (the best in hot places), open SunSpec Modbus. Slow start in backup mode (20 to 90 seconds). Low DC oversizing. Read this together with the read and write limit above. Its default warranty is short unless you register in its cloud portal.

RCT Power. German. Community integration for Home Assistant. Reading is solid, writing is more limited. A smaller ecosystem.

GoodWe. Official Home Assistant integration. Local read and write. The common complaint is warranty service: owners report ignored tickets. Fine with a strong local installer who handles claims.

SolaX. Controllable but the most fiddly of the usable platforms: a known Home Assistant integration with quirks, and it needs the correct dongle (a type that supports Modbus).

Fenecon (OpenEMS). Philosophically very open. The free app is read-only. Write control (over Modbus or REST) needs a paid, one-time licence. If you shortlist it, ask the installer to include the write-access app in the offer.

Victron. Fully local, MQTT and Modbus, open system. Seamless backup and off-grid ability. More do-it-yourself, a lower peak efficiency, and an AC-coupled style. A tinkerer's choice.

SolarEdge. Best for heavily shaded roofs (module-level). Only one Modbus connection at a time, and many battery settings locked to the installer app. Choose it for real shade, not for openness.

Cloud-leaning examples. EcoFlow, Viessmann Vitocharge, Hoymiles and Tesla Powerwall are app and cloud first. Local write control is missing or limited. Felicity can be controlled by wired RS485 with community tools, but it is DIY and the user base is small. Good hardware in some cases, but the wrong control model for a local setup.

Ranking for openness

From most open to most closed, as a rough guide:

Victron > Sungrow, SMA, Sigenergy > Huawei, Kostal, Fronius, RCT, GoodWe > SolarEdge (locked settings) > SolaX (quirks) > Fenecon (paid write) >> Viessmann, EcoFlow, Felicity, Tesla (cloud).

Do not over-optimise

Between the top options, differences in efficiency and reliability are a few percent. This is smaller than the difference a good installer, correct sizing and a cool installation spot make. Choose from the top tier and spend your energy on installer quality and commissioning.

What to do at commissioning

• Confirm that Modbus TCP is enabled and that you receive the access details and the register documentation.
• Check that the right dongle is fitted if one is needed.
• Confirm that using Modbus does not affect the warranty.
• Use a network cable, not Wi-Fi, to the inverter.
• Test the failsafe. Pull the cable of your controller mid-charge and watch whether the inverter returns to its own logic.
• Check the evcc compatibility list for your exact model.

Check list

Checked in July 2026. Software and support change. Verify the exact model before you buy.

What you want to do

With a heat pump the useful control tasks are:

• Read data: temperatures, power, mode.
• Switch modes: for example a boost for hot water when solar power is plenty.
• Change set points: a higher room or hot water target for a while.
• Limit power when the grid asks for it (§14a).

Heat pumps are more closed than inverters. Their makers worry about the machine being damaged by wrong commands. So many give you only the safe parts.

EEBus is not Home Assistant control

EEBus sounds like the open answer. Look closer.

1. EEBus is not automatically usable in Home Assistant. The open reference library says it is a base for building support in different systems. It cannot be dropped in as is. Other systems need their own interface.
2. EEBus gives no set points. In the reference implementation, values such as temperatures and the ability to change them are not exposed. The one known EEBus to Home Assistant bridge, for one maker, can do only power limiting (§14a style) and power measurement, not heating control. So EEBus is about grid dimming and metering, not about heating with set points.
3. Real-world reports. One user with a popular heat pump and its EEBus-capable module reported that several tries with EEBus and Modbus TCP failed. There was no option to enable Modbus in the app, and the EEBus search found nothing. A request in the evcc project for exactly this device is still open.

The deepest control of a heat pump often goes through the maker's local API. That API is usually open only to partner energy managers, not to end users.

EEBus is still useful when a manager and a heat pump from compatible families talk to each other. An example is a smart home manager from one brand that speaks EEBus with a heat pump module that also speaks it. PV surplus heating then works. But then that manager controls the heat pump. Your own Home Assistant does not. It solves the energy problem. It does not solve your wish to control everything yourself.

A ranking by openness

This ranking is from a check in July 2026. Treat it as a snapshot.

1. Stiebel Eltron and Tecalor (Stiebel group), with the ISG gateway. Official Modbus TCP and a core Home Assistant integration. The only platform in the check with a first-party path. Other models in the same group share this.
2. Wolf (CHA, FHA). EEBus, natively supported by evcc (can be dimmed, can report power). It was not in the offers of the real project, but good to know.
3. Vaillant. EEBus plus a community bridge, but only for power limiting and measurement.
4. Buderus (Bosch). EEBus is offered by the LAN module since March 2025, but there is no known route into Home Assistant. A community project (EMS-ESP) exists.
5. Viessmann. The ViCare API is cloud-based. Home automation goes through the cloud.
6. Panasonic. Local control only with a community box (HeishaMon), a do-it-yourself solution.

Other makers may sit anywhere on this scale. Always ask for the exact model and check.

A fallback for any heat pump

Almost every heat pump has SG-Ready terminals (chapter 21). evcc has a device type, sgready-relay, that closes the two SG-Ready contacts with a cheap smart relay (for example two Shelly relays) for "boost" and "dim". A Shelly energy meter measures the power. This works locally with brands that offer no open interface, including Buderus, Viessmann and Panasonic.

• Cost: roughly 100 € in hardware, plus installation.
• Limit: it is coarse, only boost and dim, no set points.
• Result: no heat pump is completely uncontrollable. You lose precision, not control.

Ask your installer or an electrician who wires it. The task often falls between trades.

What to ask and check

• Which interface does the offered heat pump have? Modbus, EEBus, SG-Ready, or cloud only?
• Is the gateway or LAN module included in the price? (For some brands it is a paid extra.)
• Can my home automation system control it, or only the maker's own manager?
• Are the SG-Ready contacts wired, and by whom?

Check list

Solar surplus to the heat pump

When the sun gives more than the house and battery need, you can steer the surplus into the heat pump. It heats the hot water tank and the buffer with your own power, instead of exporting it for a small payment. It is one of the most valuable automations.

Three ways

1. SG-Ready. The simple, universal option: a coarse "heat more now" signal (chapter 21). Almost every heat pump understands it.
2. EEBus. A newer EU standard between heat pump or wallbox and an energy manager. More detailed than SG-Ready. See chapter 23 for its limits.
3. A home energy manager or Home Assistant. It sees PV, battery and heat pump together and divides the surplus.

Do your brands talk to each other?

The inverter knows when there is surplus. The heat pump must receive that. Across brands this is the crux. Some PV makers can output an SG-Ready or EEBus signal directly. Otherwise a manager or Home Assistant does the bridging: it reads the surplus from the inverter and sends the signal to the heat pump. That is why local control of both devices is valuable.

What you can realistically automate

• Hot water on surplus. Heat the tank a bit higher when there is lots of PV. A hot water tank is a "free battery" for heat.
• Buffer boost. Run the heating buffer slightly warmer on surplus to save grid power later.
• No dead-heading. The heat pump still needs its minimum flow (chapter 16). Surplus heating means "more into the buffer", not "close the radiators".

The winter reality

In winter, when the heat pump needs the most power, solar surplus is smallest. The coupling shines in spring, summer and autumn, especially for hot water. In deep winter, efficiency and a dynamic tariff help more.

Dynamic tariffs

A dynamic tariff (for example from Tibber, aWATTar or Rabot) gives you hourly prices that follow the wholesale market. Prices are often low at night and at midday and high in the morning and evening.

You need:

• a smart meter system (iMSys) (chapter 19),
• a controller that can shift battery charging and heat pump running into cheap hours, either the maker's own energy management or a local one like evcc or Home Assistant,
• a battery whose grid charging can be controlled. Not every brand allows this equally well.

Battery arbitrage

With a dynamic tariff the battery becomes a price buffer, not only a solar buffer.

• Cheap hours: charge the battery from the grid (or run the heat pump).
• Expensive hours: live off the battery.
• Sunny hours: store solar surplus as usual.

Who is the battery manager?

By default the inverter manages the battery in self-consumption mode. The moment you send charge commands, you take over (chapter 22). Think about:

1. serving self-consumption sensibly, not only arbitrage,
2. keeping a minimum charge level,
3. the failsafe: what happens if your controller stops? Ideally the inverter returns to self-consumption on its own.

Is it worth it?

The saving depends on the price spread and your use. In many cases it is a few hundred euros a year. That can justify the setup, but it is not automatic. Grid charging adds wear to the battery, so a good controller weighs the saving against the cycle cost.

Also, in cheap hours many loads bunch together: car, heat pump, battery. Check the house connection (chapter 19) and let the energy manager limit the total load.

Charging the electric car

A wallbox with solar surplus charging can start charging when the sun gives enough power and slow down when clouds come. Useful settings:

• A minimum solar mode (charge from surplus only) for daytime parking.
• A fast mode when you need the car soon.
• A price-based mode for night charging in cheap hours.

Provision the cable, the circuit and the network connection for an 11 kW wallbox now, even if the car comes later (chapter 8). Vehicle-to-home, where the car supplies the house, is coming. A system that already manages a wallbox is a plus.

Check list

You now know the parts. Part 5 is about the offers. This chapter is the first of seven steps: get offers that you can actually compare.

Prepare a one-page brief

Installers give better offers when they get the same clear facts. Write one page and send it to everyone.

The house

• Address area, year built, living and heated area, number of people.
• Roof: type of covering (tile, slate, sheet), direction of each side, tilt, shade, free area. Photos help.
• Heating today: oil or gas, age, yearly use (litres or kWh), hot water through the heating, circulation line.
• Radiators or underfloor heating. Type of piping if you know it (one-pipe or two-pipe).
• Insulation and windows, done and planned.
• Meter cabinet and main fuse: photos, age.

Your use and plans

• Yearly electricity use today (from your bill).
• Planned electric car, and when.
• Planned renovation work: roof, windows, facade, and the dates.

What you want

• PV size goal (for example 17 to 18 kWp, or "fill the roof").
• Battery goal (for example 10 to 12 kWh usable, modular).
• Heat pump: sized from a heat load calculation for the renovated house.
• Whole-house 3-phase backup, yes or no.
• Wallbox, yes or no, and which size.
• Control: local control with Modbus, no cloud lock-in, dynamic tariff ready.
• Funding: you want the installer to help with the application.

This is also your first check on yourself. If you cannot write a line, you may need to read a chapter again.

Who to ask

• Local installers. Good for service in year 8. Ask for references nearby.
• Municipal utilities (Stadtwerke). They can be a neutral local benchmark.
• National and online providers. Often fast and polished. Their offers may include their own cloud platform.
• Independent advisors and energy consultants. Useful as a cross-check, usually not as installers.
• Comparison portals and lead brokers. They pass your request to firms. You will receive calls from companies you did not choose. They add nothing that you cannot do yourself.

Aim for five to eight serious offers. Fewer gives you little to compare. More becomes unmanageable. Remember that some firms do both PV and heat pump and some only one.

Different kinds of offers

You will see several types. Know which one you hold.

Dates and old offers

Offers have a validity date. Many expire in 2 to 3 weeks. Some are old: an offer from 2023 or 2024 is stale. Prices and models change. If a good firm's offer is old, ask for a new quote at your target size.

The site visit

For anything beyond a rough estimate, the installer should visit. They check the roof, the meter cabinet, the cellar, the radiators and the place for the heat pump. A firm that never visits, or does the whole thing from photos, should explain how they check the points above.

Keep a tracker

You will lose track otherwise. Use a simple table. The workbook that accompanies this project has an offer tracker with these columns:

• Company and contact person
• Type (PV, heat pump, both)
• Status (rough estimate, offer received, expired)
• In my comparison? (yes or no)
• Date of last contact
• Next action
• Notes

Write the next action every time. For example "ask for new quote at 17 kWp", "get Heizlast", "answer by Friday".

Pressure and deadlines

Time pressure is a classic sales tool. "5 % discount if you order within 14 days." "Offer ends tomorrow." The discount may be real, but do not let a clock force you to sign before you have compared the others. A fair firm will extend or give you the same price later. If it does not, that is information.

Also watch out for large deposits and unclear financing. Slow down, and compare like for like.

Next step

Once the offers arrive, the real work starts: comparing them fairly. That is the next chapter.

The comparison table

The method is simple. Make a table with one column per offer and one row per feature. Fill each cell from the offer document. Where the offer says nothing, write "not stated". Every "not stated" is a question for the installer.

The rows come in groups. These are the groups used in the comparison workbook of this project.

For a PV offer

For a heat pump offer the rows are similar: price lines, unit (model, kW, heat load basis, SCOP and JAZ, COP at A-7, design flow temperature, modulation, refrigerant, monobloc or split), noise and siting, hot water and hydraulics (tank, 60 °C, buffer, diagram, radiator check, balancing), controls (SG-Ready, EEBus, Modbus, cloud), scope and service (removal of the old system, electrical work, funding application, lead time, warranty, maintenance, service network, exclusions) and control.

The chapters of Parts 2 to 4 give you what "good" looks like for most of these rows.

Three prices

Every offer needs three price lines. Mixing them is the number one source of bad comparisons.

1. Net price (without VAT).
2. Gross price (with VAT).
3. Price after funding (after grants).

Remember:

• PV and battery on a private house carry 0 % VAT in Germany at the time of writing. Net and gross are then the same.
• Heat pumps carry 19 % VAT.
• A funding rate is a hope until the approval arrives. Most offers bill you whatever the outcome. Budget the gross price and treat the grant as a rebate you may get (chapter 32).
• In one example, a heat pump offer showed 35,000 € gross and "about 22,000 € after funding". The second number assumed a rate that was not guaranteed. Compare gross against gross, and funding separately.

Price per kWp

For PV, compute € per kWp = total price divided by array size. For a complete system with battery and wallbox in southern Germany in 2026, 1,500 to 1,800 € per kWp is a typical range (chapter 9).

The price per kWp helps you find outliers. It is not a valid comparison until the offers have the same scope.

Scope-matched price

Here is the heart of an honest comparison. Take one offer as your baseline and list everything it includes: array, battery size, backup, wallbox device, replacement tiles, scaffolding, SPD, earthing, new meter cabinet, grid registration. Then take each other offer and add the price of what is missing, using the quote or a fair estimate. This gives a scope-matched price.

Example (numbers are illustrative):

Offer A looked 3,200 € cheaper. After matching the scope, it is 2,450 € more expensive. This is a very common pattern. In the real project, one offer with a headline price of about 25,300 € became about 29,000 to 31,000 € after the missing items were added.

Where a number is an estimate, mark it. Do the same for heat pumps: removal of the old oil boiler and tank, buffer tank, electrical work, energy consultant, funding application.

Weights and scores

Not all rows matter equally to you. Give each important row a weight from 1 to 5. Then rate each offer for that row, for example 0 to 5, and multiply. Add up to a weighted score.

This is optional. Many people reach a clear answer without it. But a score is useful in two ways: it forces you to say what matters, and it helps you explain a decision to a partner.

Typical high-weight rows: price (scope-matched), local control, backup, warranty, installer service, completeness of scope.

Check the arithmetic

Add up the line items yourself. In the real project, a renegotiated offer was checked line by line, and all totals and the discount agreed. Check it anyway. Mistakes in totals, VAT and discounts happen.

A caution about "verified" and "researched"

The comparison workbook marks some control cells with a note: they were researched from manufacturer and community sources, not taken from the offer. Do the same. Keep what the offer states separate from what you found. Verify the second kind with the installer in writing.

A low headline price does not mean a low final price. This chapter lists where the extra cost usually hides. Use it as a scan sheet on every offer.

Items that are often missing or optional

"Bedarfsposition" or "Eventualposition" in German offers means an item that may be added if needed. Treat it as a cost you will probably pay.

Contract and payment traps

Front-loaded payments. Some plans ask for 60 % after delivery of the modules and the rest before commissioning, or even 90 to 100 % before the system runs. You then have paid before everything works. Look for a plan such as 10 % at signing, 60 % after roof work and 30 % after commissioning. Avoid paying 100 % before the AC installation.

Retention of title. The company owns the system until the last payment. Insure it in the meantime. Often your building insurance covers it for little.

Deposits. A large deposit is a risk if the firm fails. Check the company age, references and, if possible, the commercial register.

Cancellation right (Widerruf). You usually have 14 days. Do not give it up before the scope-critical points are confirmed in writing (price, scope, clauses like "up to X kWp").

"Up to" clauses. "Up to 17 kWp" can later become 15 kWp. Fix the module count in the contract.

Time pressure. "Order within 14 days for 5 % off." The discount may be real. Do not sign before you have compared (chapter 25).

Lease, rent or PPA. The monthly figure hides the total cost. Check the term, the price at the end, who owns the hardware and who controls it.

Subcontracting and the warranty chain. The seller, the installer and the warranty holder may be three different companies. Ask who you call in year 8.

Expired and old offers. An offer past its date, or two years old, is not a price. Ask for a new one.

Rough estimates. A "preview" or "initiative" offer without a site visit is not a final price.

Technical limits in the small print

Inverter array limit. The inverter may be limited by the maker to a maximum array size. In one real offer a 10 kW inverter was limited to 15 kWp, while the offer had 17.9 kWp. A second inverter was added for extra money.

Undersized inverter. An 8 kW inverter on a 15.6 kWp array clips a lot.

Battery price per kWh. One offer charged 748 € per kWh for a battery, while another had about 415 € per kWh. Compute the price per usable kWh.

Expansion limits. Some batteries can be extended only within two years, or only with the same module type. Ask for the price of an extension and the conditions.

Warranty conditions. A longer warranty may need registration in the maker's cloud portal, or an annual service contract. Check the conditions.

Local control behind a payment. Some functions (for example write access or dynamic tariff charging) need a paid licence or a subscription. Ask if everything you need is in the price.

One kind of backup, not the one you assumed. See chapter 18.

Small module counts. If your roof can hold 41 modules and the offer uses 32 although the inverter could carry more, ask why.

How to deal with an unclear item

1. Write it in your table as "unclear".
2. Ask the installer in writing: included? price? who does it?
3. Put the answer into your scope-matched price (chapter 26).
4. If a firm will not give a straight answer, mark it as a risk.

This chapter takes one real offer from the project and reads it slowly. Company names are left out. It is a model for how to read your own offers.

The situation

An installer sent one offer in two versions. Both had the same array: 14.4 kWp of glass-glass modules, the same roof mounting, an 11 kW wallbox, earthing and a new meter cabinet. They differed only in the inverter and battery brand.

Variant 2 is about 1,586 € cheaper and has 1 kWh more storage. On paper, the choice looks easy. Now we check.

What is true

The modules are genuinely good. They are n-type TOPCon, glass-glass, fire class A, hail class 5, with a roughly 30-year warranty on that product line. The brochure's language about "maximum efficiency and durability" is, for once, broadly justified. Still, ask for the exact datasheet for the temperature coefficient and the warranty percentages (chapter 5).

Both batteries are LFP. That is the safe, long-life choice (chapter 7). Variant 2's battery is verified at about 6,000 cycles to 70 % health, with a 15-year warranty that includes labour and call-outs. That is a strong warranty.

Both inverters are DC-coupled 3-phase hybrids with backup. That is a good architecture for a new build (chapter 6). The first brand has the stronger open and local energy management story. The second brand is more app and cloud led, so you should confirm that it exposes local Modbus access if integration matters to you (chapters 20 to 22).

What is brochure language

Bifacial on a tiled roof. The module is bifacial, but on a normal tiled pitched roof the rear side adds little. Treat bifacial as nice to have. Do not pay extra for it here (chapter 5).

What to question

The bottom line

If you want this installer's system, variant 2 is better value on paper: cheaper, more storage and a verified 15-year battery warranty. Variant 1's edge is its open energy management and an established German storage record, which may matter if you plan home automation. You must decide which of the two you weight higher. This is exactly what the weight column in your comparison table is for (chapter 26).

The offer is also not out of line against the planning range: roughly 1,060 kWh per kWp per year in southern Germany, and about 1,500 to 1,800 € per kWp all-in (chapters 3 and 9). It looks reasonable, pending the questions above.

A second lesson: a misleading headline

In the same project, another installer sent an offer with a headline of 25,316 € for 17.02 kWp. It looked like a bargain. Reading it showed:

• the meter cabinet renewal was not included and not priced,
• earthing, AC surge protection, backup, SG-Ready preparation and the wallbox were all "optional items",
• the company was newly formed after a takeover, and made no statement about local control.

After adding the missing items, the scope-matched price was about 29,000 to 31,000 €, or about 1,727 € per kWp. That was more expensive than two other offers in the comparison. The cheap headline was the result of leaving things out.

Your own routine

For every offer you receive:

1. Read all pages, including the small print and the general terms.
2. Write down what is stated. Write down what is missing.
3. Check the specs against the tables in Parts 2 and 3.
4. List the questions (chapter 29).
5. Add the cost of missing items to get a scope-matched price (chapter 26).
6. Note the validity date, the payment plan and the warranty conditions.

How to use the list

After you have filled your comparison table (chapter 26), you will have many "not stated" cells. This list turns them into questions.

1. For each offer, mark the questions that are not answered in the offer.
2. Send only those questions. Do not ask what the offer already says.
3. Ask for answers in writing, by email. Keep them.
4. If a question is answered with a vague word, ask for a number.

You can also use the list as a pre-check before an installer visit.

PV questions

Heat pump questions

Questions for single offers

Beside the master list, add one or two questions that come from your reading of that offer, for example:

• "Your offer states a notice that the Steuerbox and wiring to the heat pump may be at my cost. Please give a price."
• "Please include the paid write-access licence for local control in the offer and show the price."
• "The offer says 'selected circuits' for backup. Which circuits exactly? Is a quote for whole-house backup possible?"
• "The inverter datasheet limits the array to X kWp, but your plan has Y kWp. How do you solve this?"

German version (copy and paste)

Many installers work in German. This is the master list in German. Send only the points that are open.

Photovoltaik

• F1 Wir möchten die Dachbelegung auf die maximal sinnvolle Größe erweitern. Bitte aktualisieren Sie das Angebot. Wie viele Module und kWp passen auf unser Dach?
• F2 Bitte bieten Sie einen 3-phasigen Hybrid-Wechselrichter mit 12 bis 15 kW AC an, passend zu 17 bis 18 kWp. Welches Modell schlagen Sie vor? Wie groß darf der Generator maximal sein?
• F3 Speicher: 10 bis 12 kWh nutzbar, modular erweiterbar. Welche Ausbaustufen gibt es, und was kostet eine spätere Erweiterung pro Modul?
• F4 Notstrom: Ist eine automatische, 3-phasige Ganzhaus-Notstromversorgung enthalten? Welche Zusatzhardware ist erforderlich (Umschalteinrichtung, separate Absicherung, Verdrahtung), was kostet sie? Wie lang ist die Umschaltzeit? Ist das System schwarzstartfähig?
• F5 Eigensteuerung: Ich betreibe Home Assistant bzw. evcc und möchte Wechselrichter, Speicher, Wallbox und Einspeisung über Modbus TCP bzw. lokale API steuern (lesen und schreiben), ohne Cloud-Zwang. Bitte bestätigen Sie schriftlich: (a) Modbus TCP bzw. die lokale API wird bei der Inbetriebnahme aktiviert und dokumentiert übergeben, (b) die Nutzung beeinträchtigt die Garantie nicht, (c) ich erhalte Zugangsdaten und die Registerdokumentation.
• F6 Sind Metall-Ersatzziegel an allen Dachhaken enthalten, um Ziegelbruch zu vermeiden? Falls nicht: Aufpreis?
• F7 Ist ein Erder (Tiefenerder oder Fundamenterder) inklusive Verkabelung bis zum Zählerschrank und Anschluss an den Potentialausgleich enthalten? Falls nicht: Aufpreis?
• F8 Unser Zählerschrank ist alt. Ist die Erneuerung nach DIN VDE-AR-N 4100 im Angebot enthalten? Kosten?
• F9 Wir wollen einen dynamischen Stromtarif nutzen. Bereiten Sie den Zählerplatz für ein intelligentes Messsystem (iMSys) vor und stimmen Sie sich mit dem Messstellenbetreiber ab? Genügt ein Zweirichtungszähler, und misst Ihr System den Netzpunkt zusätzlich lokal?
• F10 Erweiterung: Falls wir zunächst nur einen Teil des Dachs belegen, was würde eine spätere Erweiterung um 3 bis 4 kWp kosten (Gerüst, Elektrik, erneute Netzanmeldung, ggf. zweite Vergütungstranche)? Was empfehlen Sie? Bitte beide Varianten beziffern.
• F11 Wartung: Welche jährlichen Inspektions- oder Wartungskosten fallen an bzw. empfehlen Sie? Welche weiteren laufenden Kosten sollen wir einplanen?
• F12 Sind Sie auf die angebotenen Marken festgelegt, oder können Sie alternativ andere Wechselrichter (z. B. Sungrow, SMA, Fronius) liefern und in Betrieb nehmen?
• F13 Steuerbox: Wird für die Einspeisesteuerung (EEG §9) bzw. für §14a eine Steuerbox benötigt? Ist sie im Angebot enthalten bzw. vorbereitet?
• F14 Service: Wer ist in 8 bis 10 Jahren mein Ansprechpartner bei Störungen: Ihr Unternehmen, ein Subunternehmer oder der Hersteller? Welche Reaktionszeiten und Garantieabwicklung bieten Sie?
• F15 Wie viele unabhängige MPP-Tracker sind vorgesehen, und wie werden die Strings auf die Dachflächen verteilt?
• F16 Ist Überspannungsschutz enthalten: SPD Typ 1+2 auf der AC-Seite und Typ 2 auf der DC-Seite? Falls nicht: Aufpreis?
• F17 Ist die Erneuerung des SLS-Schalters im Angebot enthalten, insbesondere im Zuge der Zählerschrank-Erneuerung? Welche Nenngröße sehen Sie vor?
• F18 Unser Haus ist Baujahr 1972. Bitte prüfen Sie Hausanschluss und SLS für den gleichzeitigen Betrieb von Speicher-Netzladen, Wallbox (11 kW) und künftiger Wärmepumpe. Reicht die Anschlussleistung, ist ein Lastmanagement vorzusehen, oder wäre eine Verstärkung durch den Netzbetreiber nötig (Kosten, Dauer)?

Wärmepumpe

• W1 Bitte senden Sie uns die Heizlastberechnung (DIN EN 12831) für den sanierten Zustand des Hauses.
• W2 Warum ist diese Gerätegröße richtig? Was passiert am kältesten Tag, und welche Rolle spielt der Heizstab?
• W3 Haben Sie unsere Heizkörper raumweise bei der Auslegungs-Vorlauftemperatur geprüft? Wie hoch ist sie? Ist unsere Heizung eine Einrohrheizung?
• W4 Ist der hydraulische Abgleich (Verfahren B) enthalten?
• W5 Wie hoch sind SCOP und die berechnete JAZ bei unserer Vorlauftemperatur? Wie hoch ist der COP bei A-7/W35?
• W6 Welches Kältemittel? Monoblock oder Split? Wie groß ist der Modulationsbereich?
• W7 Welche Speichergröße für Warmwasser? Erreicht der Verdichter 60 °C für den Legionellenschutz, oder übernimmt das ein Heizstab? Wie wird die Zirkulationsleitung behandelt?
• W8 Sind Pufferspeicher und Hydraulikschema enthalten?
• W9 Bitte nennen Sie Schallleistung und Schalldruck in einer genannten Entfernung inklusive Nachtmodus und bestätigen Sie schriftlich, dass der geplante Standort die TA Lärm am Nachbarfenster einhält.
• W10 Welche lokale Schnittstelle hat dieses Modell (Modbus, EEBus, SG-Ready)? Ist das Gateway bzw. LAN-Modul im Preis enthalten? Sind die SG-Ready-Kontakte verdrahtet, und von wem?
• W11 Sind Ausbau von alter Heizung und Öltank, Elektroarbeiten und der neue Stromkreis enthalten? Ist der Energieberater für den Förderantrag enthalten?
• W12 Wie lang sind Geräte- und Verdichtergarantie? Ist ein Wartungsvertrag nötig? Wer wartet das Gerät in Jahr 8?

A red flag is not proof of a bad firm. It is a reason to ask. Some can be fixed with one email. If you collect several red flags in one offer, slow down.

The ten flags

1. Vague component names. "Premium modules", "high-quality inverter", "powerful battery" with no brand and no exact model. Without a model number you cannot check a single spec. Do: ask for brand, exact model and datasheet for each main part.

2. Prices before and after funding mixed up. The after-funding number is shown as if it were the price, or a funding rate is assumed that you may not get. Do: ask for net, gross and after-funding separately (chapter 26).

3. The two warranties mixed up. A "25-year warranty" that is only the output curve, with a short product warranty hidden below. Do: ask for both, in years (chapter 5).

4. Only COP at A7/W35 for the heat pump. The flattering mild-weather number. Without SCOP, JAZ and the COP at A-7 you cannot judge winter running cost. Do: ask for SCOP, calculated JAZ and the cold-day COP (chapter 11).

5. No heat load calculation and no radiator check. The heat pump is sized by rule of thumb, and nobody checked that your radiators work at low flow temperature. This is the classic recipe for a disappointing, expensive retrofit. Do: ask for the heat load calculation and the radiator check (chapters 12 and 13).

6. "Backup-ready" with no backup hardware. The inverter or battery could do backup, but the box and wiring are not in the scope. In a blackout you get nothing. Do: ask exactly what is included (chapter 18).

7. Missing balance of system. No surge protection, no proper metering, undersized cables, generic racking. Corners cut where you cannot see them. Do: go through the check list in chapter 8.

8. High-pressure or "today only" pricing. Artificial urgency, large deposits, or financing, lease or PPA terms that hide the total cost. Do: slow down. Compare like for like. Ask for the total cost over the whole term.

9. Subcontracted installation with an unclear warranty chain. The seller, the installer and the warranty holder are different companies. In year 8 you may not know whom to call. Do: ask who installs, who services and who honours the warranty.

10. Cloud-only ecosystem. Monitoring and control locked to the maker's cloud, with no local or open interface. A risk of lock-in and of outage, and a barrier to home automation. Do: ask for local Modbus or a documented local API (chapters 20 to 23).

More signs to watch

• Expired or very old offers. Not a price.
• A "preview" or rough estimate treated as final.
• Payment before the system runs (chapter 27).
• "Up to X kWp" without a fixed module count.
• Offers that do not mention the meter cabinet, although the house is old.
• Inverter limited by the maker to a smaller array than the one planned.
• Brand mismatch, such as great modules paired with a budget inverter and battery with unclear warranties and poor integration. Good parts wasted on weak ones.
• Numbers that do not match. For example, a report that says 23 kWp while the offer says 10.8 kWp. Ask which is right.
• A company that is newly formed after a takeover, with no record you can check.
• Cheaper only because things are left out.

Mistakes buyers make

It is fair to admit that buyers also make errors. Some from the real project:

• Comparing price per kWp before matching scope. A cheap offer looked best until the missing items were added.
• Claiming too much from the brochure. A platform was described as fully open, then the notes found that local write control needed a paid licence.
• Relying on an old note. A statement that a heat pump brand had "no official local protocol" was out of date. A later check found an EEBus interface, though it still gave no path into Home Assistant. Dates and sources matter in your own notes.
• Sizing on the old consumption. The old oil use pointed to a large heat pump. The renovated house needed far less.

Keep a short log of what you got wrong and how you fixed it. It makes the next decision better.

Start with your priorities

A good decision starts with a short list of what matters to you, in order. Write it down before you look at the final prices. Here is one real example, from a house owner with a 1972 house:

1. Local control and open protocols. Modbus or EEBus, no cloud lock-in.
2. One coordinated system. PV, battery, heat pump and later an electric car controlled together, ideally with Home Assistant.
3. Sound hardware and value.

Yours may be different. For example: lowest total cost, whole-house backup, the nearest service partner, the best warranty, or the least work for you. There is no wrong list. There is only a list you did not write down.

Then give each priority a weight (chapter 26). When you compare, the weights show which offer fits your list.

Tiers: a simple way to rank

The comparison workbook sorted offers into three groups:

• A - meets the priorities well, with only small questions open.
• B - good, but a clear gap exists (control, size, scope, price).
• C - does not fit: stale, closed to local control, much more expensive after matching scope, or not a real offer.

Ranking is not a ranking of "good" and "bad" companies. It is about fit for one household.

Build the shortlist

1. Remove offers that are expired, rough estimates or templates.
2. Remove offers that fail a must-have (for example no local control when you require it).
3. Match scope for the rest (chapter 26).
4. Keep two to four offers.
5. For each, send the open questions (chapter 29) and get answers in writing.
6. Ask the best two for a new, final offer with all points included, at your target size.

Example of a pairing that fits the priorities above: a PV and battery system on an open, locally controllable platform, plus a heat pump on a platform that offers a first-party local interface, tied together in Home Assistant. PV surplus goes to the battery and then to the heat pump buffer, all on local Modbus and SG-Ready, aware of dynamic tariffs. No vendor cloud is needed. (The brand notes in chapters 22 and 23 show which families fit.)

Check the practical points

Before you sign, look at:

• Service. Is the installer near you? Who services in year 8 to 10? A local electrician's firm a few kilometres away can be a strong argument.
• Completeness. Does the final offer include everything in your scope-matched list?
• Size. Does the array fill the roof sensibly? Does the inverter match? (Chapters 6 and 9)
• Heat pump engineering. A real heat load for the renovated state, an emitter check, hydraulic balancing, design flow temperature, SCOP at that temperature and the noise at the boundary. Reject rule-of-thumb sizing.
• Control. Written confirmation that Modbus or the local API is enabled and that the warranty is not affected.
• Funding. The sequence (chapter 32) and who does the application.
• Payment plan. No large payment before the system runs.

Negotiate scope, not only price

Instead of only asking for a discount, ask to include missing items. In the real project, one installer renegotiated and moved to a version with metal tiles, backup and a complete new meter cabinet all included, at a 9 % discount, and the total came out clearly better than the first version. Check the new total line by line.

Renovation in parallel

If the house is being renovated at the same time, the order of work matters. See chapter 36: roof first, wiring and conduits while walls are open, and a shared scaffold.

Do not over-optimise

Between the best options, the differences are small compared to the quality of the installer, the correct sizing and good commissioning. At some point more research stops paying. When you have a shortlist of offers that fit your priorities and answer your questions in writing, choose the one whose firm you trust most.

The last checks

• Read the contract and the terms, not only the offer.
• Check that the module count, the models and the price match the offer.
• Check the validity of any discount and the cancellation right.
• Keep a copy of every document and email.
• For heat pumps: do not sign before the funding sequence is clear, or insist on a clause that makes the order depend on the funding approval (chapter 32).

What if funding is uncertain?

Treat the heat pump grant as unconfirmed until you have the approval. Budget the gross price. Most offers bill you whatever happens.

As of October 2026. Rates and rules change, and the heating funding was reformed during 2026. Check the figures and the steps with KfW or your specialist firm before you apply.

KfW pays a grant when you replace a fossil heating system with a heat pump. The grant can be up to about 70 % of the eligible costs, capped. The order of the steps is strict. If you miss it, the grant is gone.

The six steps

1. BzA (Bestätigung zum Antrag, confirmation for the application). The specialist firm, or an energy efficiency expert, issues it. Only a registered firm can. It unlocks everything.
2. Apply in the "Meine KfW" portal, with the BzA, before you sign the binding contract.
3. Approval (Bewilligungsbescheid) from KfW.
4. With the approval, go to your house bank for the KfW supplementary loan (chapter 33).
5. Installation, final invoice, and you pay.
6. BnD (Bestätigung nach Durchführung, confirmation after completion) from the firm after you pay. With it you draw the grant.

A short way to remember: the BzA gets you to approval, the BnD gets you to payout.

The trap: apply before you order

The most common costly mistake is to sign the contract first and apply later. That is too late.

The fix is a conditional clause in the order: the contract depends on the funding approval. This keeps the order "apply, approve, then contract and start" even though you formally placed the order. Many specialist firms offer this clause themselves. Ask for it.

Note that "start of works" for funding usually means delivery or installation, not the conditional signature. Confirm this with the firm or the energy consultant.

The rates, as understood in 2026

The grant is built from parts. The rates below are the ones used in the project in 2026. They can change, so check the current figures.

The eligible cost is capped (around 30,000 € for the first residential unit).

Example: a household above the income threshold with an old oil boiler and an R290 heat pump gets about 35 % base plus 20 % speed, about 55 %. If the eligible cost is 30,000 €, the grant is about 16,500 €. In the project, offers showed after-funding amounts in this range, for example 44,995 € turning into about 28,495 € own share after about 16,500 € grant.

Treat the grant as unconfirmed until you hold the approval. Many offers bill you the full price whatever the outcome.

What changes the amount

• An old oil or gas boiler helps with the speed bonus.
• Several residential units with rental contracts can raise the cost cap. Tell the firm early, so the BzA shows it.
• The income bonus applies below the threshold.
• A second unit or an older system can matter. Report it early.

What is funded elsewhere

• Windows and insulation are not funded here. They go through BAFA (BEG EM), a different pot, which can combine with the KfW heating grant.
• PV modules and inverters are generally not subsidised. They get 0 % VAT instead.

Hydraulic balancing

Funding requires hydraulic balancing, usually Verfahren B, and certain efficiency conditions. Make sure the offer contains them (chapter 12).

As of 2026. Terms and income limits can change. Check with your bank and KfW.

Besides the grant there is a low-interest supplementary loan (Ergänzungskredit) to finance your share. You do not apply for it directly at KfW. You apply through your house bank.

358 and 359

• KfW 358 depends on income. Below a household income limit (of the order of 90,000 € taxable income) the interest rate is particularly favourable.
• KfW 359 is the standard version without an income limit.

Both go up to 120,000 € per residential unit and complement the grant. They finance the part the grant does not cover.

How it fits the sequence

The loan needs the grant approval. So the order is: BzA, application, KfW approval, and then with the approval to your house bank. The bank passes the KfW loan on.

Tip: you can prepare the talk with the bank before the approval comes, so it is faster once you have it. Ask early which documents the bank needs.

What the bank checks

• Creditworthiness and proof of income, perhaps a land register extract, as with other loans.
• For 358, the income limit, which decides the cheaper rate.
• Whether the bank passes on KfW loans at all. Not every bank does it actively. If yours is unwilling, ask another bank that does.

If KfW does not fit

If the supplementary loan does not cover everything, or you want to finance more, ask about other loans. Often the rest can be a normal loan or be added to an existing mortgage. Ask for the interest rate, the term and the conditions, and compare them.

As of 2026. Municipal programmes change often, often every year. Always check your municipality's current guideline.

Many municipalities pay for a heat pump, and sometimes a battery, on top of KfW. That is extra money. But rules can cap the total.

What it looks like

One example from a municipality near Munich, as of July 2026: for a heat pump 25 %, up to 2,000 €. Other municipalities have their own rates. Such programmes are usually combinable with KfW and BAFA. They are meant to stack.

The warning: programmes change

The same programme a year earlier also paid a battery grant (15 %, up to 500 €). The 2026 version removed it. Never trust an old brochure or a blog post. Get the current guideline and the current forms.

The 60 % cap

The most important rule: the total funding from all sources (KfW plus the municipality) often may not exceed 60 % of the eligible costs. The KfW grant alone can be high. So:

• With a high KfW rate, the municipal grant may be reduced, or drop to zero.
• Ask the climate office of your municipality how they calculate the 60 % cap when KfW is high. This decides if the full amount arrives.

The right order

The municipal heat pump grant usually needs the KfW approval notice when you apply. So it comes after the KfW approval. As with KfW, the application must also be submitted before the work starts. The conditional clause in your contract covers both.

What to do

1. Ask for the current forms early, from the municipality website or the climate office.
2. Clarify the 60 % question in advance.
3. Apply after the KfW approval, before installation starts, with the KfW notice and the offer.
4. After the work, submit the final documents on time. Often these are the KfW payout notice and the invoices, within a few months.

As of 2026. Rules change. Check with the funding body if in doubt.

Funding is valuable, but it is full of traps of order and timing. Tick these off.

Before you order

• Apply before you order. Never sign the binding contract before the funding application, or put the conditional clause into the order confirmation.
• Secure the BzA. Make sure the firm issues it and you know what it needs from you.
• Report a second residential unit or an old oil boiler early. They can change the amount.

When you stack grants

• Mind the 60 % total cap. KfW and municipality together may use up to 60 % of eligible costs. With high KfW funding the municipal grant can shrink.
• Keep the order. The municipal application usually comes after the KfW approval and before construction.

At the contract

• Cancellation right (Widerruf). Do not waive your 14-day right before price, scope and "up to X kWp" clauses are confirmed in writing.
• Payment plan. Plans are often front-loaded: for example 60 % after delivery and 40 % before commissioning. You pay before everything runs. Combined with retention of title, the firm owns the system until the last payment. Insure the system meanwhile.

After the installation

• Get the BnD from the firm, after you have paid. With it you draw the grant.
• Register in the Marktstammdatenregister (MaStR), within about one month of commissioning, yourself. It is free and online.
• Municipal final documents. Submit them on time. Often these are the KfW payout notice and the invoices.

In one line

(1) Apply or put in a condition before ordering. (2) Secure the BzA. (3) Check the 60 % cap. (4) Waive the cancellation right only after the scope is confirmed. (5) Think about prepayment and retention of title. (6) Get the BnD. (7) Register the system yourself.

Between "offer accepted" and "system running" there are several steps, often in parallel for PV and heat pump. Knowing the order helps you plan dates and avoid surprises.

The PV sequence

1. Engineering (Projektierung). The firm makes the detailed plan: string layout, components, meter cabinet. You get it for approval.
2. Your approval (Freigabe). An important checkpoint. Check that module count, battery, backup and clauses like "up to X kWp" match the offer before this becomes binding.
3. Order confirmation based on the final plan.
4. Grid registration with the grid operator, and at the same time ordering of the hardware.
5. Delivery and installation. Usually roof mounting first (weather permitting), then the connection to the house.
6. Completion notice to the grid operator. The utility inspects and installs a new meter. This can take weeks.
7. You register the system in the Marktstammdatenregister within about a month.

The heat pump sequence

For a funded heat pump: order confirmation with a conditional clause and the BzA, then you apply to KfW, then approval, then installation (often in autumn), then final invoice, you pay, BnD and you draw the grant (chapter 32). The municipal grant slots in after the KfW approval (chapter 34).

If you renovate at the same time

When the house is open anyway, a few small things now save a lot later.

• Roof first. If the roof is part of the renovation, do covering and insulation before the PV mounts go on. Coordinate the work so the scaffolding is shared between the roof work and the PV job. Several PV offers exclude scaffolding or charge for it.
• Run network cable now. Lay LAN cable to the inverter, the heat pump and the wallbox. Local control depends on it. Do not rely on Wi-Fi. Offers sometimes charge a surcharge when no cable is there.
• Prepare for the wallbox. Lay the conduit and a 3-phase circuit for a future wallbox, and think about a larger meter cabinet.
• Add surge protection. With the walls open and a new cabinet, adding full AC and DC protection is cheap now and painful later.
• Flow temperature. Aim for the lowest design flow temperature the renovation allows. Insulate, and where you can, enlarge or change radiators or add underfloor or wall heating (chapter 12).
• Pipe system. If you have single-pipe heating, convert to two-pipe or upgrade the radiators before or with the heat pump.

What to prepare

• Internet at the inverter and heat pump, by cable.
• Clear space where the inverter and battery go, cool and dry (chapters 6 and 7).
• Forward all letters from the grid operator to your installer. Utilities do not always inform the firm.

Watch the payments

Expect front-loaded instalments. Remember retention of title, and insure the system during construction. Often your building insurance covers it for little (chapter 35).

Commissioning checklist

At commissioning, go through the following with the installer.

• The system runs as built. Check each string, the battery and the inverter display.
• Modbus TCP or the local API is enabled. You have the access data and the documentation (chapter 22).
• The right dongle is fitted, if needed, and the connection is wired by cable.
• The failsafe works. Disconnect your controller and see that the inverter returns to its own logic.
• Backup: test a real power cut with the installer, and check which circuits stay on. Does the heat pump run (chapter 18)?
• Heat pump: heating curve set low, flow temperature checked on a cold day, hydraulic balancing documented, SG-Ready contacts working.
• Surge protection, earthing and meter cabinet are done as stated in the offer.
• Documents: datasheets, warranty papers, commissioning report, wiring diagram, hydraulic diagram, funding documents.
• You have registered in the MaStR, and the grid operator has installed the new meter.

Units and basics

Solar and battery

Heat pump

Backup, grid and control

Offers and money

The blank workbook

The comparison method in Part 5 works best in a spreadsheet. We made a blank workbook with the same structure as the one used in the project. It contains no prices or company names.

Download the blank workbook (.xlsx)

It has these sheets:

• Start here: how to use it, with a colour key.
• PV offers and Heat pump offers: one column per offer, one row per feature, a weight column and a score block.
• Education sheets: modules, inverter, battery, heat pump, mounting and balance of system, control and home automation. Each row says why the spec matters, and what good, acceptable and red flag look like.
• Glossary and red flags.
• Offer tracker: a blank table for all firms.

Fill the amber cells. Keep net, gross and after-funding prices apart.

Checklist 1: before you ask for offers

• I wrote a one-page brief (house, roof, heating, use, plans, wishes).
• I have photos of the roof, the meter cabinet and the heating room.
• I know my yearly electricity use and my oil or gas use.
• I decided my top three priorities.
• I started an offer tracker.
• I know which renovation work is planned, and when.

Checklist 2: before I sign

• The offer is final and valid, not a rough estimate.
• Brand, exact model and datasheet are named for modules, inverter, battery and heat pump.
• I have net, gross and after-funding prices, and I budget the gross price.
• Meter cabinet, tiles, scaffolding, surge protection, earthing, backup and wallbox device are included, or priced.
• The scope-matched price is calculated and compared.
• A heat load calculation for the renovated state, a radiator check and hydraulic balancing are in the heat pump offer.
• I have the written answers to my open questions.
• The payment plan does not make me pay before the system runs.
• I know who services the system in year 8.
• For funding: the BzA is arranged and the order has a conditional clause.
• I read the contract and terms.

Checklist 3: commissioning

• The system runs as built, all strings and the battery are online.
• Modbus TCP or the local API is enabled. I have the access data and the documentation.
• The network connection is by cable.
• The failsafe works.
• A real power-cut test was done. I know which circuits are backed up.
• Heating curve is set low. Hydraulic balancing is documented.
• SG-Ready contacts are wired and tested.
• I have all documents: datasheets, warranty papers, reports, diagrams.
• I registered in the MaStR. The new meter is installed.
• I ordered the BnD.

Checklist 4: control readiness

Use this on any inverter, battery or heat pump you consider.