Why UK systems are sized for winter

The single number that makes or breaks a UK design: peak sun hours.

A "peak sun hour" is one hour of perfect, full-strength sunshine. UK reality:

Summer: 3.5–4.5 peak sun hours/day (more in the south)
Winter: 0.8–1.2 peak sun hours/day — and under 0.8 in northern Scotland

That's a 4–5× swing. A system sized for the yearly average works beautifully in May and dies in December, exactly when you want the lights on.

So the honest method: take your daily energy need, divide by your *winter* sun hours, add ~25% for system losses (cable, controller, battery round-trip), and that's your panel wattage. Yes, you'll have a huge surplus in summer — that's the price of a system that works in January.

Two ways to soften it: tilt panels steeply (60–70°) to catch the low winter sun, and add a day or two of battery autonomy to ride out the greyest stretches.

From EcoPowerful — plain-English DIY solar & wind guidance for UK homes, with a free instant system plan builder. Guidance is general; 230V fixed wiring always needs a qualified electrician. Last updated 2026-06-13.