Solar Panel Calculator
A solar array almost never produces its headline rating, because that figure is measured under laboratory conditions. This calculator applies realistic sun hours and system losses.
Formula
Daily kWh = panel watts × number of panels × peak sun hours × efficiency ÷ 1,000
- Peak sun hours is not daylight hours; UK averages roughly 2.5–4
- Efficiency allows for inverter, wiring, temperature and dirt losses
10 × 400 W panels, 4 sun hours, 85% efficient: 13.6 kWh a day
How Output Is Estimated
Daily output (kWh) = Panel watts × Number of panels × Peak sun hours × Efficiency ÷ 1,000
Ten 400 W panels with 4 peak sun hours at 85% system efficiency give 400 × 10 × 4 × 0.85 ÷ 1,000 = 13.6 kWh a day.
What Peak Sun Hours Mean
Peak sun hours are not daylight hours. One peak sun hour is an hour at 1,000 W per square metre — full midday summer sun. A long overcast winter day may deliver under one peak hour despite eight hours of daylight.
UK averages run roughly 2.5 to 3.5 across the year, with a large seasonal swing: a system generating 20 kWh on a June day may produce 2 kWh in December. Annual totals matter far more than any single day.
Where the Losses Go
System efficiency of 80–85% accounts for inverter conversion, cable losses, dust and shading, and the fact that panels lose output as they heat up — ironically, a hot day can produce less than a cool bright one.
Orientation matters too: in the UK, south-facing at around 35 degrees is optimal, while east or west-facing arrays typically lose 15–20%. Panels also degrade slowly, usually warranted at around 80% of original output after 25 years.
Quick Reference
| System | Sun hours | Per day | Per month | Per year |
|---|---|---|---|---|
| 10 panels (4 kW) | 3 | 10.2 kWh | 306 kWh | 3,723 kWh |
| 10 panels (4 kW) | 4 | 13.6 kWh | 408 kWh | 4,964 kWh |
| 16 panels (6.4 kW) | 4 | 21.8 kWh | 653 kWh | 7,942 kWh |
| 25 panels (10 kW) | 4 | 34.0 kWh | 1,020 kWh | 12,410 kWh |
Frequently Asked Questions
- How much electricity will my solar panels generate?
- Multiply panel wattage by the number of panels, peak sun hours and system efficiency. Ten 400 W panels in the UK typically yield 10 to 14 kWh on a good day and far less in winter.
- What are peak sun hours?
- Hours equivalent to full midday sun at 1,000 W per square metre, not hours of daylight. The UK averages roughly 2.5 to 3.5 across the year.
- Do panels work in winter?
- Yes, but output falls sharply with shorter days and lower sun angles. December output can be a tenth of June output.
- Does heat improve output?
- No, the opposite. Panels lose efficiency as temperature rises, so a cool bright day often outperforms a hot one.