You don't need an engineering degree to guess your system size. You need one number off your electricity bill, and a couple of simple steps. This is the same method MCS installers start from.
What's your yearly electricity use?
Find your kilowatt-hours per year. It's on your annual statement, or you can add up twelve months of bills. A typical UK home uses about 2,700 kWh (Ofgem's 'medium' figure, dropping to roughly 2,500 kWh from July 2026). Most homes fall somewhere between about 1,800 and 4,100 kWh, but yours is the only number that really matters.
How many panels does that number turn into?
A modern 400–450W panel makes roughly 350 to 450 kWh a year in the UK, depending on your roof and region. Divide your yearly usage by that number, and you'll have a rough panel count. A typical home lands around 8 to 12 panels, about a 3.5–4.5 kWp system. Shade, roof pitch, and which way your roof faces can nudge that number from there.
The quick version
- Use your real yearly kWh, not a rough average.
- One UK panel makes roughly 350–450 kWh/year.
- Most homes need 8–12 panels (≈3.5–4.5 kWp).