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