Solar panel size calculator

Peak sun hours are not clock hours of daylight. They are the equivalent hours of strong midday sun that produce the same energy as a whole day. A dull UK day can have many daylight hours but only one or two peak sun hours.

The sun is low, days are short, and cloud is common. Southern England in November to February is often around one peak sun hour. Scotland and the north are lower still. Winter camping usually needs a bigger array, more battery days, or hook-up.

This page uses one loss figure for dirt, angle, controller and cable. An MPPT controller usually wastes less than PWM, especially in cold or poor light. If you know you will use PWM, raise the losses a little. The watt figure is still only a planning size.

This calculator does not measure roof space. A typical rigid 100 W panel is about 1.2 m by 0.55 m; 200 W and 400 W panels are larger. Hatches, vents, aerials and walkways steal space. Use the panel counts as a shopping hint, then check the actual roof.

Rigid glass panels usually last longer and stay cooler. Flexible panels can suit a curved roof or a hire van where you must not drill, but they often run hotter and wear sooner. The watt size here is the same either way — pick the type that fits the roof and how long you will keep the van.

A vehicle charger only works while the engine is running, and many vans only put a modest charge into the leisure battery. It helps on travel days. It will not cover a parked weekend the way solar can. Plan solar for the days you stay put; treat driving charge as a bonus.

No. Solar puts energy in during the day. The leisure battery covers the night and dull spells. Size the battery for the days you want to last without charging, then size solar to put that daily use back. Use both tools. They share the same daily watt-hours on this device.

If you plug in most nights, you can plan a smaller array or skip solar. Use this page for the nights you stay off-grid. A small panel can still top phones and a fridge on a sunny day, but hook-up is doing the heavy charging.

The default 25% is a simple allowance for dirt, a less-than-ideal roof angle, the solar controller, and cable losses. It is not a lab measurement. Raise it if the roof is shaded or flat in winter; lower it a little if panels are clean, well aimed, and on an MPPT controller.

Treat it as a planning size, then round up to panels you can actually buy and fit. Weather, shade, fridge run-time and roof angle all change the real figure. The 100 W, 200 W and 400 W counts are examples only — not a shop. Leave a margin and use a qualified installer for safety-critical work.