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.
A reproducible calculation with complete inputs
For a reproducible solar panel calculator example, use the metric input system and enter Yearly electricity use: 6000 kWh; Share of use to cover: 100 %; Peak sun hours per day (yearly average): 4 h; Panel power: 430 W; System losses: 14% (typical: inverter, wiring, heat, dirt); Usable roof area: 40 m². These settings describe one specific scenario, not a new default for every job. The result follows the method and allowances already explained above. Keep the entire set of values when comparing it with a hand calculation; omitting a selected material or an allowance can change the result even when the dimensions match. The following rows show the unrounded calculation values alongside the precision used by the interface.
| Result | Calculation value | Displayed value | Unit |
|---|---|---|---|
| Panels needed | 12 | 12 | |
| Required system size | 4.77859190825 | 4.78 | kW |
| Installed size with whole panels | 5.16 | 5.16 | kW |
| Estimated yearly output | 6478.896 | 6479 | kWh |
| Panel footprint | 24 | 24.0 | m² |
| Panels that fit the roof | 20 | 20 |
How should I prepare these entries?
Yearly electricity use: Add up 12 months of bills. Typical: 3,000–4,500 kWh for a European home, 10,000–11,000 kWh in the US. Share of use to cover: enter the value in %. Peak sun hours per day (yearly average): About 2.7 in London, 3.2 in Paris or Bern, 4.5 in Madrid, 4 in Chicago, 6 in Phoenix. Panel power: On the datasheet (Wp). Residential panels are mostly 400–450 W. System losses: choose among 14% (typical: inverter, wiring, heat, dirt), 10% (optimised, cool climate), 20% (hot climate, partial shade). Usable roof area: Only the sunny slopes, minus edges, chimneys and skylights. Choose the unit system before transferring your measurements. A depth, length, area and coverage can have different field units even within one system. Read each label individually rather than entering the same unit convention in every box. Optional openings or allowances should describe your actual scenario instead of being silently left at a sample value. The worked inputs above identify exactly which selections produced the table.
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.