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Enter the size of the system in kWp, the average peak sun hours per day at your location and the performance ratio. You get the production per day, month and year, and what it is worth. It is an estimate for planning, not a yield guarantee.

Enter your values

Peak power of all panels together: number of panels × watts per panel ÷ 1000.
Where do I find this?

Multiply the number of panels by the watt-peak of one panel, then divide by 1000: 12 panels × 420 Wp = 5.04 kWp.

The offer or the inverter documents also state the kWp.

Yearly average of the daily solar energy in kWh/m². Look up your location.
Where do I find this?

Peak sun hours equal the daily solar irradiation in kWh per m². The European Commission's free PVGIS tool (re.jrc.ec.europa.eu/pvg_tools) gives it for any location.

Averaged over the year it is often around 2.5 to 3.5 in Central Europe, higher in the south. 3 is an example.

Share of the ideal output that remains after losses. Often 75 to 85 %.
Where do I find this?

The performance ratio covers inverter, cable, temperature and shading losses. Well-built systems are often around 80 %.

Use a lower value for shading or a poor roof angle.

What a kWh is worth to you: the electricity price you avoid buying, or the feed-in tariff.
Where do I find this?

For power you use yourself, use your electricity price per kWh. For power you feed into the grid, use the feed-in tariff.

The Solar Payback Calculator mixes both.

Only changes the symbol; no exchange rate is applied.

Result

Enter your values to see the result.

Production per year
– kWh
Per month
– kWh
Per day
– kWh
Yield per kWp and year
– kWh/kWp
Value per year
–

Want this tracked for you? UtilityControl logs your meters and costs automatically.

How it is calculated

  • Energy per day (kWh) = system size (kWp) × peak sun hours × performance ratio ÷ 100
  • Per month = per day × 365 ÷ 12; per year = per day × 365
  • Yield per kWp = peak sun hours × 365 × performance ratio ÷ 100

Example

A 5 kWp system with 3 peak sun hours and an 80 % performance ratio makes 5 × 3 × 0.8 = 12 kWh a day, about 4,380 kWh a year.

What this estimate leaves out

Real production varies with the weather from year to year, with roof angle and direction, shading and panel ageing. For a precise yield of your roof use a tool such as PVGIS or ask your installer for a simulation.

kWp and kWh: what is the difference?

kWp (kilowatt-peak) is the rated power of the panels, measured under standard test conditions: 1,000 W of sunlight per m² at a cell temperature of 25 °C. kWh is energy: how much electricity the system actually delivers over a day or a year.

So there is no fixed factor that turns kWp into kWh. You need two things from your site: how much sun reaches the panels, and how much is lost on the way to the socket. Together they give the yield per kWp, and kWh per year = kWp × kWh per kWp.

Two ways to convert kWp to kWh

With peak sun hours, as in this calculator: kWh per day = kWp × peak sun hours × performance ratio.

With the yearly solar irradiation on the panels, the form used in engineering standards such as IEC 61724: kWh per year = irradiation on the panels (kWh/m² per year) × kWp × performance ratio. Both give the same result, because the yearly irradiation divided by 365 equals the daily peak sun hours.

Example for Germany: Fraunhofer ISE gives about 1,270 kWh/m² a year on south-facing panels tilted 20 to 25°, which is about 3.5 peak sun hours a day. A 5 kWp system with a performance ratio of 85 % then makes about 1,270 × 5 × 0.85 ≈ 5,400 kWh a year, or about 1,080 kWh per kWp.

kWp to kWh per year: quick reference

Production per year for common system sizes. Pick the column that matches the yield per kWp at your location.

Yearly production in kWh by system size and yield per kWp
System size800 kWh/kWp1,000 kWh/kWp1,200 kWh/kWp1,600 kWh/kWp
1 kWp800 kWh1,000 kWh1,200 kWh1,600 kWh
3 kWp2,400 kWh3,000 kWh3,600 kWh4,800 kWh
5 kWp4,000 kWh5,000 kWh6,000 kWh8,000 kWh
8 kWp6,400 kWh8,000 kWh9,600 kWh12,800 kWh
10 kWp8,000 kWh10,000 kWh12,000 kWh16,000 kWh
15 kWp12,000 kWh15,000 kWh18,000 kWh24,000 kWh

Typical yields: about 800 kWh/kWp in the UK (median load factor of feed-in tariff installations 2024/25: 9.2 %), about 1,080 kWh/kWp for a new south-facing system in Germany, and up to about 1,600 kWh/kWp in southern Spain or North Africa (Fraunhofer ISE). Older, shaded or poorly oriented systems make less.

Sources: UK Government: Feed-in Tariff load factor analysis 2024/25 · Fraunhofer ISE: Recent facts about photovoltaics in Germany · European Commission JRC: PVGIS solar irradiation and yield tool

kWh to kWp: how big must the system be?

Turn the formula round: kWp = kWh per year ÷ yield per kWp. To produce 4,000 kWh a year where 1,000 kWh per kWp is realistic, you need about 4 kWp, for example 10 panels of 400 Wp.

Producing as much as you use over a year does not mean you use all of it yourself, because most solar power comes at midday and in summer. The Solar Payback Calculator shows what the split between own use and feed-in means for the money.

Written and reviewed by QLines, Cologne · Last reviewed

Guide: how big should your solar system be?

FAQ

Frequently Asked Questions

How do I convert kWp to kWh?

Multiply the system size by the yield per kWp at your location: kWh per year = kWp × kWh per kWp. Per day it is kWp × peak sun hours × performance ratio. There is no fixed factor, because the sun differs from place to place.

How many kWh does 1 kWp produce per year?

About 800 kWh in the UK, about 1,000 to 1,100 kWh for a new south-facing system in Germany, and up to about 1,600 kWh in very sunny regions such as southern Spain.

How many kWh does a 10 kWp system produce?

About 8,000 kWh a year in the UK, around 10,000 to 11,000 kWh on a good south-facing roof in Germany, and up to about 16,000 kWh in very sunny regions.

Is kWp the same as kW?

Not quite. kWp is the DC power the panels deliver under standard test conditions. The AC power the inverter feeds into the house is usually a little lower, and on most days the panels stay below their peak rating.

How much electricity does a solar panel produce?

It depends on the system size and the sun at your location: kWh per day = kWp × peak sun hours × performance ratio. A 5 kWp system with 3 peak sun hours and 80 % ratio makes about 12 kWh a day.

What are peak sun hours?

The number of hours per day with the full 1 kW/m² of sunshine that would give the same energy as the real day. It equals the daily solar irradiation in kWh per m². Look up your location in PVGIS.

What is kWp?

kWp (kilowatt-peak) is the output of the panels under standard test conditions. It is the usual way to state the size of a solar system.

What is the performance ratio?

The share of the ideal output that is left after losses in the inverter, cables, temperature and shading. A good system often has about 75 to 85 %.

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