How the solar panel calculator works
A solar array is sized in kilowatts, but you pay for electricity in kilowatt-hours. The link is peak sun hours: the number of hours per day, averaged over the year, that the sun delivers the equivalent of 1,000 W per square metre on the panels. One kilowatt of panels produces about one kilowatt-hour for each peak sun hour, minus losses in the inverter, cables, dust and heat.
System kW = yearly kWh × share ÷ (peak sun hours × 365 × (1 − losses))The result is divided by the panel wattage and rounded up to whole panels. Footprint is based on a modern monocrystalline module producing about 215 W per square metre, so a 430 W panel measures roughly 2 m² (21.5 ft²), close to the common 1.72 × 1.13 m format.
Worked example
A household uses 6,000 kWh a year and lives where the yearly average is 4 peak sun hours. With 14% losses, one kilowatt of panels makes 4 × 365 × 0.86 = 1,256 kWh a year, so covering 6,000 kWh takes 6,000 ÷ 1,256 = 4.78 kW. With 430 W panels that is 11.1, so 12 panels, an installed 5.16 kW that should produce about 6,480 kWh a year. The twelve panels cover roughly 24 m², and a 40 m² south-facing slope holds up to 20 of them, leaving room for the fire-access margins some areas require. The same home in London at 2.7 hours would need about 17 panels.
Sun and yield by location
| Location | Peak sun hours (year average) | Typical yield per kW installed |
|---|---|---|
| London, Hamburg | 2.7–3.0 | 850–950 kWh/yr |
| Paris, Bern, Munich | 3.0–3.4 | 950–1,100 kWh/yr |
| Chicago, New York | 3.8–4.3 | 1,200–1,350 kWh/yr |
| Madrid, southern Italy | 4.6–5.2 | 1,450–1,650 kWh/yr |
| Los Angeles, Phoenix | 5.3–6.0 | 1,650–1,900 kWh/yr |
Figures assume a panel facing the equator at a good tilt. East or west roofs lose about 10–20%, and a flat installation loses more in winter. Free public tools such as NREL PVWatts or the EU PVGIS service give an exact figure for your address.
Things that change the answer
- Future loads. An electric car adds roughly 2,000–3,500 kWh a year and a heat pump 3,000–5,000 kWh. Sizing only for today's bill can leave you short in two years.
- Shade. A chimney or tree shading one panel can pull down a whole string; microinverters or optimisers limit the damage.
- Export rules. Where exported power earns little, covering 60–80% of use with a smaller array often pays back faster than 100%.
- Winter versus summer. In northern Europe a panel makes four to six times more in June than in December, so full yearly coverage still means buying grid power in winter.
- Roof and wiring checks. An array adds about 12–15 kg per m². Have an installer or engineer confirm the roof structure, and use a qualified electrician for the grid connection and permits.
Frequently asked questions
How many solar panels do I need to power a house?
A US home using 10,500 kWh a year with 4.5 peak sun hours needs about 7.4 kW, which is 18 panels of 430 W. A European home using 4,000 kWh at 3 hours needs about 4.2 kW, or 10 panels.
How many kWh does a 400 W solar panel produce?
Multiply by peak sun hours and losses: at 4 hours and 14% losses a 400 W panel makes about 1.4 kWh a day, or 500 kWh a year.
How much roof space do I need for solar panels?
Each modern 400–450 W panel covers about 2 m² (21 ft²), so a 5 kW array needs roughly 23–25 m² (250–270 ft²) before edge setbacks.
What are peak sun hours?
The daily solar energy at your site expressed as hours of full 1,000 W/m² sun. Five hours of weak morning and evening light might count as only two peak sun hours.
What size solar system for 6,000 kWh a year?
At 4 peak sun hours and 14% losses you need about 4.8 kW, which is twelve 430 W panels; at 3 hours it rises to about 6.4 kW.
Last reviewed: 2026-09-27 · How we calculate