How Many Solar Panels Do I Need? A UK Sizing Guide
Most UK installers quote a system size without showing their working. Here is the methodology to check whether what you are being quoted is the right size for your home.
Start with your annual consumption
Find your annual electricity usage in kWh on your energy bill or via your smart meter's in-home display. The UK average is around 3,500kWh per year for a 3-bedroom home. If you have an EV, add approximately 2,000–3,500kWh depending on annual mileage. If you are planning to get a heat pump, add 3,000–5,000kWh.
How much of that can solar cover?
Solar cannot cover your entire annual consumption from direct generation — the timing mismatch means you generate more than you need in summer and less in winter. A realistic target for self-consumption (using your own generation directly) is 40–55% of annual consumption, depending on system size and whether you have a battery.
With a battery: 55–70% self-consumption is achievable.
Without a battery: 30–45% self-consumption is typical.
The sizing calculation
Rule of thumb: 1kWp of south-facing panels generates around 850–950kWh per year in central England (higher in the south, lower in Scotland). So:
3,500kWh consumption × 40% target self-consumption = 1,400kWh needed from solar
1,400kWh ÷ 900kWh per kWp = 1.56kWp minimum
But you also want to generate surplus to export and to cover higher consumption periods, so most households install 3–6kWp.
How many panels is that?
Modern 400W panels (the current standard) mean:
3kWp = 7–8 panels
4kWp = 10 panels
5kWp = 12–13 panels
6kWp = 15 panels
Each panel needs roughly 1.8m² of roof space. A 4kWp system needs about 18m² — which fits comfortably on most standard semi-detached roofs.
Should you size up or down?
Size up if: you have or plan an EV; you have space on the roof; you are likely to add a battery later (more generation means more to store). The marginal cost of an extra panel or two is low relative to the install cost.
Size down if: your roof is significantly shaded in summer; you are on a budget and want the shortest possible payback period; you are selling the property within 5 years.
The roof constraint
Before the sizing calculation becomes relevant, confirm your usable roof area. A south-facing pitch with no chimneys, Velux windows, or satellite dishes is the best case. A north-facing roof is the worst — not economically viable. East/west split roofs work but generate 15–25% less than south-facing equivalents. An installer shading report (typically using PVSol or similar software) is part of any competent survey.