Weather forecast energy management means using the forecast for the next hours, not only today's readings, to decide when a heat pump runs, when hot water is heated and how much solar power to expect. The two numbers that matter most are the outdoor temperature and the solar radiation: they tell you how much heat the building will need and how much electricity the roof will produce.
This guide explains which forecast data an energy management system uses, how it turns that data into PV yield and heating estimates, which automation rules pay off, and how EnergyOS by QLines uses the 48-hour forecast of the German weather service for exactly these jobs.
Key Takeaways
- An energy management system that only reacts to live readings acts late: the surplus or the cold night is already there. A forecast lets it act before.
- Two forecast values do most of the work: outdoor temperature (heating demand) and solar radiation (PV yield). Cloud cover explains the radiation.
- Typical forecast-driven rules: pre-heat before a cold spell and leave hot water to the sun when a sunny day is coming.
- Comparing expected PV energy with measured PV energy is a simple fault check: a sunny forecast with low output points to shading, soiling or a defect.
- EnergyOS loads the 48-hour forecast for the location of your systems every hour, shows coldest hour, expected PV yield and heating demand, and runs forecast rules on top of the price and PV-surplus rules.
What Is Forecast-Driven Energy Management?
Forecast-driven (or predictive) energy management uses weather forecasts to plan energy use ahead of time. Instead of waiting until the grid meter shows a PV surplus or the room temperature drops, the system looks at the next 12 to 48 hours and shifts flexible loads, such as heating, hot water and EV charging, to the hours where energy is cheapest, greenest or most plentiful.
Live readings stay essential: they show what is happening now and confirm that a command arrived. The forecast adds the time dimension. Together they answer a question a reading alone cannot: should the heat pump work now, or later? If you are new to the topic, our guide What Is an Energy Management System (EMS/HEMS)? explains the three jobs of an EMS: measure, decide and control. The forecast belongs to the second job.
Which Weather Data Does an Energy Management System Need?
For a building with a heat pump and solar panels, three hourly values cover most decisions: the outdoor temperature at 2 meters, the solar radiation on a horizontal surface, and the cloud cover. Wind, humidity and precipitation matter for special cases, such as air-source heat pump defrosting or snow on panels, but they are rarely needed for everyday rules.
| Forecast value | Unit | What it tells the energy management system |
|---|---|---|
| Outdoor temperature | °C | How much heat the building needs; how efficient an air-source heat pump will run; when the coldest hour comes |
| Solar radiation (global, horizontal) | W/m² | How much electricity PV panels will produce per installed kWp; which hours are "sunny" |
| Cloud cover | % | Why radiation is high or low; useful for explaining a grey day |
Definitions used in this article. kWp (kilowatt peak) is the rated power of a PV system under standard test conditions. Performance ratio is the share of the theoretical yield a real PV system delivers after losses in the inverter, wiring, heat and dirt; values around 75 to 85 % are typical. Heating degree hours add up, hour by hour, how far the outdoor temperature falls below a heating limit: the more degree hours, the more heat a building needs. They are the hourly cousin of heating degree days, which the U.S. Energy Information Administration describes as comparing the mean outdoor temperature with a standard base temperature (EIA, Degree days).
How Does a Forecast Become a PV Yield Estimate?
A simple and robust PV estimate multiplies the forecast radiation by the installed power and a performance ratio: hourly radiation (W/m²) ÷ 1,000 × kWp × performance ratio = kWh in that hour. Summed over the next 24 hours, it tells you whether tomorrow is a 3 kWh day or a 30 kWh day.
Example: a 10 kWp system, a forecast with eight hours of 500 W/m² on average, and a performance ratio of 80 %. Each sunny hour gives 0.5 × 10 × 0.8 = 4 kWh, so the day brings about 32 kWh. A grey November day with 50 W/m² for six hours brings about 2.4 kWh. The estimate ignores roof tilt and orientation, so it will not match the meter to the kilowatt-hour, but it reliably separates a sunny day from a grey one, which is what a rule needs.
How Does a Forecast Become a Heating Estimate?
Heating demand follows the outdoor temperature. Counting heating degree hours below 15 °C over the next 24 hours gives a single number that rises on cold days and falls on mild ones. A day averaging 3 °C gives 24 × 12 = 288 degree hours; a mild day at 10 °C gives 120. The building needs roughly 2.4 times as much heat on the cold day, all else being equal. Together with the coldest hour, this tells you when to heat ahead and how much the heat pump will run.
EnergyOS with demo systems and the real DWD forecast for München on 14 November 2026 (chart times in UTC): coldest hour, expected PV yield, heating demand and the 48-hour temperature and radiation curve.
Which Automation Rules Use the Weather Forecast?
The most useful forecast rules move heat, because heat is easy to store: in the building's walls and floors, and in the hot water tank. Electricity is harder to store without a battery. Two rules cover the most common situations in a house with a heat pump and PV.
1. Pre-heat before a cold spell
When the lowest outdoor temperature forecast for the next 12 hours drops below a threshold, for example −5 °C, the heat pump is asked to run more now, while it is still milder. Air-source heat pumps work more efficiently at milder temperatures, and the building stores the heat for the cold hours. With an SG Ready heat pump, "run more" means the mode recommended on; when the cold has passed, the mode goes back to normal. The heat pump's own controller still protects comfort and limits.
2. Leave hot water to the sun
When the PV yield forecast for the next 24 hours is high, there is no need to heat hot water from the grid in the morning: the sun will deliver the energy at midday. The rule lowers the grid-heated hot water target, for example to 45 °C, and the PV surplus raises it later. On grey days the normal target, for example 55 °C, applies. This rule pairs well with a PV-surplus rule, which switches the heat pump on when the panels export more than a threshold. Our article on solar and heat pump controllability explains the surplus side in detail.
3. Check PV output against the forecast
Not every use of the forecast is a control rule. If the forecast says the panels should have produced 18 kWh by noon and the meter shows 8 kWh, something is wrong: a shaded string, heavy soiling, snow, or an inverter fault. A daily comparison of expected and measured PV energy turns the forecast into a cheap fault detector. A large gap is a reason to look, not proof of a defect, because the estimate does not know the roof's orientation.
| Rule | Forecast condition | Action | Needs |
|---|---|---|---|
| Pre-heat before a cold spell | Lowest temperature in the next 12 h at or below the threshold (°C) | Heat pump "recommended on", back to "normal" afterwards | Controllable heat pump (for example SG Ready) |
| Leave hot water to the sun | PV yield forecast for the next 24 h at or above the threshold (kWh) | Hot water target 45 °C instead of 55 °C | Heat pump with a hot water target; PV system with its kWp |
| PV expected vs measured | Measured PV energy clearly below the forecast (for example under 70 %) | Hint to check shading, soiling or faults | PV power readings |
How EnergyOS Uses the Weather Forecast
EnergyOS loads the weather forecast for the location of your energy systems: the MOSMIX forecast of the German weather service, Deutscher Wetterdienst (DWD), for the nearest forecast point, delivered through the open Bright Sky service and refreshed every hour. On the energy page, EnergyOS shows:
- the coldest and warmest hour of the next 24 hours,
- the expected PV yield for the next 24 hours, from the installed kWp of your PV systems and an 80 % performance ratio, with the number of sunny hours,
- the heating demand as degree hours below 15 °C,
- a 48-hour chart of outdoor temperature and solar radiation,
- for real PV systems, PV today, expected versus measured, with a hint when the output is clearly below the forecast.
On the Automation page, the two forecast rules sit next to the existing rules for PV surplus, cheap electricity on the day-ahead market and the grid operator's limit under §14a EnWG. Every rule follows the same safety logic: it switches a control when its condition starts, sets it back when the condition ends, changes at most every five minutes, and does nothing when it cannot see its data, for example when no forecast is available.
Demo data in EnergyOS: two forecast rules next to a PV-surplus rule. Each rule has its own threshold and can be paused or deleted.
What EnergyOS does not do: it does not replace the heat pump's own weather-compensated control, which still sets the flow temperature from the outdoor temperature. EnergyOS works one level above it, by asking the heat pump to run more or less, or by changing the hot water target, through a gateway next to the equipment (for example Home Assistant, Node-RED or evcc). The PV estimate is a planning value, not a guaranteed yield. If your equipment is not connected yet, the forecast card still shows the weather numbers for your location.
How to Set Up Forecast-Driven Rules in Five Steps
- Enter the location of your systems once: town, postcode or address. The forecast appears on the energy page.
- Add your PV system with its kWp, so the expected PV yield can be calculated.
- Connect the heat pump through a gateway that can switch SG Ready modes or set the hot water target, and check that a manual command is confirmed.
- Add one forecast rule with a cautious threshold, for example pre-heat at −5 °C, and watch it for a week.
- Compare the heat pump's runtime and grid import with the weeks before, then adjust the thresholds. Change one thing at a time.
Common Mistakes
- Over-trusting the forecast: forecasts are probabilities. Use thresholds with a margin and let rules reset when the condition ends.
- Fighting the heat pump's controller: do not override flow temperatures from outside; use the modes and targets the manufacturer provides.
- Forgetting comfort: lowering the hot water target is only sensible if the PV surplus really arrives; keep a minimum that is right for your household and hygiene requirements.
- No baseline: without a few weeks of readings before the change, you cannot tell whether a rule saved anything. Our guide on monitoring energy use shows how to build one.
Conclusion
A weather forecast turns an energy management system from reactive to proactive. The outdoor temperature tells it how much heat the building will need and when the coldest hour comes; the solar radiation tells it how much electricity the roof will deliver. With those two values, a heat pump can pre-heat while it is still mild, hot water can wait for the sun, and a weak PV day stands out as a possible fault. Start with one rule, measure the effect against a baseline and adjust the thresholds. EnergyOS shows the 48-hour forecast for your location, estimates PV yield and heating demand, and runs the forecast rules next to price and PV-surplus rules.
See the forecast for your own location
EnergyOS has a free plan and demo systems, so you can enter your town, look at the 48-hour forecast, the expected PV yield and the heating demand, and try the forecast rules before connecting any equipment. For projects, integrations or partnerships, request a meeting.
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Frequently Asked Questions
Can a heat pump use the weather forecast?
Yes. Many heat pumps already adjust their flow temperature to the current outdoor temperature. An energy management system adds the forecast: it can ask the heat pump to run more before a cold spell, through SG Ready modes or a manufacturer interface, so the building stores heat while the heat pump still works efficiently.
How accurate is a PV yield forecast?
A forecast from solar radiation, installed kWp and a performance ratio is good at separating sunny days from grey ones, but it does not know your roof's tilt, orientation or shading, so single hours can differ noticeably. It is a planning value for rules and fault checks, not a guaranteed yield.
What are heating degree hours?
Heating degree hours add up, for each hour, how many degrees the outdoor temperature is below a heating limit, for example 15 °C. Ten hours at 5 °C give 100 degree hours. More degree hours mean more heating demand, so they are a simple way to compare cold and mild days.
Where does the weather forecast in EnergyOS come from?
EnergyOS uses the MOSMIX forecast of the German weather service (Deutscher Wetterdienst, DWD) for the nearest forecast point to the location you enter, delivered through the open Bright Sky service. It covers the next 48 hours and is refreshed every hour.
Do I need a battery to benefit from the weather forecast?
No. The most effective forecast rules store heat, not electricity: in the building and in the hot water tank. A battery adds options, but pre-heating before a cold spell and heating hot water with the sun work with a heat pump and PV alone.
What happens when the forecast is not available?
In EnergyOS, a forecast rule that cannot see its data does nothing, so no equipment is switched on a guess. A recent forecast is reused for a few hours if the service is briefly unavailable, and the energy page says when the weather service did not answer.
Does the weather forecast replace live readings?
No. Live readings show what is happening now and confirm that commands arrived; the forecast shows what is coming. Good energy management uses both: the forecast decides early, and the readings check the result.
Sources
- Deutscher Wetterdienst (DWD): MOSMIX local forecasts, DWD open data server.
- Bright Sky: JSON API for the open weather data of the DWD.
- U.S. Energy Information Administration (EIA): Degree days.
- EnergyOS behavior described in this article follows the product documentation of EnergyOS by QLines, state of 14 November 2026.
Last updated: 14 November 2026.
