Kilowatt-peak (kWp) is the power a solar panel or array produces under standard test conditions (STC): sunlight of 1,000 watts per square metre striking the panel, a cell temperature of 25°C and a standard light spectrum (air mass 1.5). A 4 kWp system is one whose panels add up to 4 kilowatts under those conditions.
Why real output is lower
STC describes a bright, cool, clear moment that is rare in Britain. On most days the light is weaker, and on sunny summer days the panels heat up, which reduces their efficiency. So kWp measures size, not output. What you pay for and use is energy, measured in kilowatt-hours (kWh), and the useful figure is how many kWh each kWp generates a year.
Typical UK yield per kWp
Estimates from the European Commission’s PVGIS model (version 5.3, SARAH3 satellite data) for a 1 kWp system facing due south at a 35° pitch, with the tool’s default 14% system losses:
| Location | Estimated generation per kWp a year |
|---|---|
| Truro | about 1,070 kWh |
| London | about 1,020 kWh |
| Edinburgh | about 900 kWh |
| Manchester | about 890 kWh |
| Inverness | about 810 kWh |
East- or west-facing roofs, shallower or steeper pitches and shading all reduce these figures. MCS-certified installers must give a site-specific generation estimate using the MCS method, which accounts for location, pitch, orientation and shading.
Across the whole UK fleet, solar’s load factor was 11.1% in 2025, up from 9.2% in 2024, according to DESNZ. Its statisticians attribute the rise to 2025 being the sunniest year in their series, which began in 2001.
Read more: Solar panels for your home in the UK: costs, savings and how they work.