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 × 0.7
The 0.7 derate factor accounts for real-world losses — 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 × 0.7 = 284 kWh/year. So a household using 3,800 kWh needs 3,800 ÷ 284 ≈ 14 panels (a 5.6 kW system) for full coverage — many homes choose 10 panels (4 kW) to cover most daytime usage instead.
The same home in Scotland (840 sun hours) would need about 17 panels for full coverage, because each panel generates only ~235 kWh/year there.
Where the data comes from
- Sun hours — regional averages from NREL PVWatts / Solar Energy UK, 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 ~13p/kWh via SEG, far less than the ~26.11p/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.