How we work out your panel count
The number of panels you need depends on three things: how much electricity you use, how much sun your roof gets, and how powerful each panel is.
The formula
energy per panel / year = panel wattage (kW) × regional sun hours
Regional sun hours are PVGIS net generation figures — they already account for inverter efficiency, shading, temperature and panel degradation.
panels needed = annual usage ÷ energy per panel / year (rounded up)
system size (kW) = panels × panel wattage (kW)
roof area ≈ panels × 1.9 m² (typical 400 W panel is ~1.9 m²)
Worked example: London, 3,800 kWh/year, 400 W panels
London gets about 1,015 sun hours a year. Each 400 W panel generates roughly 0.4 × 1,015 = 406 kWh/year. So a household using 3,800 kWh needs 3,800 ÷ 406 ≈ 10 panels (a 4 kW system) for full coverage — which is what most UK homes actually install.
The same home in Scotland (840 sun hours) would need about 12 panels for full coverage, because each panel generates only ~336 kWh/year there.
Where the data comes from
- Sun hours — PVGIS 5.2 regional net generation figures in kWh/kWp (European Commission), 2026.
- Panel wattage — 400 W is the 2026 residential standard; 450–500 W are increasingly common for larger roofs.
- System cost — ~£1,600/kW installed, 0% VAT on residential solar in Great Britain until 31 March 2027.
- UK average usage — ~3,800 kWh/year for a typical 3-bed home (Ofgem typical domestic consumption).
Sizing tips
- Don't oversize if you can't use or store the surplus — exported power pays only ~6p/kWh via SEG, far less than the ~26.32p/kWh you save by self-consuming.
- If you have a heat pump or EV, size up — they add 2,000–4,000 kWh/year of demand.
- South-facing roofs generate most; east/west still hit ~80–85% of maximum.