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Solar Panel Size Calculator

Panel wattage by usage and UK sun hours

Quick answer

Panel watts = daily Wh ÷ UK peak sun hours ÷ 0.8. The UK gets ~4–5 PSH in summer but under 1 in winter, so size for spring/autumn and top up with DC-DC or hook-up through the dark months.

600Wh
Region
Season to size for
System voltage

Recommended array

400W

2.17 peak sun hours assumed

MPPT controller

34A min

Shop 400W solar kits
  • PSH figures are observed UK averages including cloud cover. Flat roof-mounted panels lose ~13% versus tilted.
  • Pair with an MPPT controller rated at least 34A, not PWM.

Affiliate disclosure: shop links are Amazon Associates links; we may earn a small commission at no extra cost to you. Guidance only — confirm against manufacturer specs before installing.

Frequently asked questions

How many solar panels do I need for a campervan?

Divide your daily watt-hours by your location peak sun hours, then by ~0.8 for real-world losses. In the UK that means roughly 200–400W for a typical build, more if you rely on solar in winter.

What are peak sun hours in the UK?

Peak sun hours (PSH) is the number of equivalent hours of full 1000W/m² sun per day. The UK gets roughly 4–5 in summer and under 1 in mid-winter, varying by region and cloud cover — not the same as daylight hours.

Do flat roof-mounted panels lose output?

Yes, around 10–15% versus tilted panels, and more in winter when the sun is low. This tool applies a ~13% flat-mount reduction so the figure is realistic.

Is solar enough for a UK winter?

Rarely on its own. From November to February most builds need a DC-DC charger (driving) or hook-up to top up. Size your panels for spring/autumn if you travel year-round.