Solar panel calculator
Sizing a solar system starts from two numbers: how much electricity you use in a year and how much one kilowatt-peak of panels produces where you live. The second number — the specific yield — varies a lot between countries, so the calculator lets you enter it.
For a specific address, the EU's free PVGIS tool estimates the output month by month with the real orientation and tilt.
Your measurements
On your electricity bill, or the annual figure ÷ 12.
How many kWh one kWp produces per year where you live. The free EU tool PVGIS gives the figure for your address, orientation and tilt.
Result
- Estimated annual use
- 4,200 kWh
- Panels needed
- 10 pcs
- Installed system power
- 4.50 kWp
A guide estimate — real output depends on the roof orientation and tilt, shading and inverter efficiency.
How the calculation works
- Annual use = average monthly use × 12.
- Output of one panel per year = panel power (kWp) × specific yield (kWh per kWp per year).
- Panels = annual use ÷ output of one panel, rounded up.
- System power (kWp) = number of panels × power of one panel.
Worked example
At 350 kWh a month, annual use is 4,200 kWh. With a specific yield of 1,000 kWh/kWp, a 450 W panel produces about 0.45 × 1,000 = 450 kWh a year, so you need 4,200 ÷ 450 = 9.3, rounded up to 10 panels — a 4.5 kWp system.
What the calculator assumes
- The specific yield assumes south-facing panels at about 30–35° without shading, with the usual inverter and cable losses included.
- A 450 W panel is about 2 m², so 10 panels take roughly 20–22 m² of roof.
When to adjust the result
- East–west orientation reduces output by 10–20%, and shade from chimneys, trees or buildings can reduce it much more.
- Most output comes in summer; in winter panels cover only part of your use, and how the difference is settled depends on your local feed-in or net-metering rules.
- If you plan a heat pump or an electric car, add their consumption before sizing.
Frequently asked questions
How many solar panels do I need for my house?
Divide your annual consumption by the annual output of one panel at your location. With 4,200 kWh a year and 450 kWh per panel, that's 10 panels.
What is specific yield?
The kWh that one kilowatt-peak of panels produces in a year at a given location and orientation. PVGIS gives it for any address in Europe and many other regions.