Short answer: this campervan solar and battery calculator adds up what your appliances use in a day, then shows the battery you need for your days off-grid (counting 85% of lithium and 50% of AGM as usable) and the solar watts needed in each UK month. Expect a few hundred watts to cover a summer day and far more than fits on a roof in winter.
What you run each day
Battery you need
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| Month | Peak sun hours a day | Solar to cover a day |
|---|
Estimates only. Sun hours are long-term averages for flat panels from PVGIS (EU Joint Research Centre, 2011 to 2020); real weeks vary a lot. Appliance figures are typical starting points: check your own labels. Guidance, not a substitute for a qualified installer.
How the campervan solar and battery calculator works
It uses the same maths as the power budget in the Roam app.
Daily use. Each appliance is watts × hours a day × the share of the time it's actually on × how many you have. The share matters for things like a compressor fridge, which cycles: the starting list assumes it runs 35% of the time. Anything on 230 V is divided by 0.9, because the inverter loses about 10% turning 12 V into mains.
Battery size. Daily watt-hours become amp-hours by dividing by the battery's nominal voltage: 12.8 V for lithium, 12 V for lead acid. That's multiplied by the days you want to go without charging and divided by how much of the battery you'll use. We count 85% of a lithium battery and 50% of an AGM or gel battery. Victron's LiFePO4 datasheet rates its batteries at 2,500 cycles at 80% depth of discharge and 5,000 at 50%, while its AGM and gel datasheet warns that repeated deep discharge badly shortens lead acid life. The 50% for AGM is a widely used rule of thumb.
Solar. Panel watts needed = daily watt-hours ÷ peak sun hours ÷ 0.75. Peak sun hours are the day's sunshine squeezed into equivalent hours of full sun, and the 0.75 allows for heat, dirt, wiring and the charge controller.
Where the UK sun figures come from
We took monthly figures from PVGIS, the European Commission Joint Research Centre's solar tool, using its satellite data (PVGIS-SARAH2) for 2011 to 2020. We used a flat, horizontal panel because that's how most van roof panels sit, and pulled six places from Cornwall to Inverness. The UK average is the mean of those six.
The swing is the thing to plan around. On those averages, a July day gives about 4.85 peak sun hours and a December day about 0.52, so the same panel makes roughly nine times as much in midsummer. Inverness in December averages 0.25. Tilting panels toward the sun helps in winter, but most of us can't do that on the road. These are averages over ten years: one grey week can be far below them, which is why a battery sized for a couple of days matters as much as the panels.
A worked example
With the starting list (fridge, lights, pump, USB, laptop, fan and a low-watt kettle), daily use comes to about 1,027 Wh. For two days on lithium that's 1,027 ÷ 12.8 × 2 ÷ 0.85, about 189 Ah, so a 200 Ah battery. Covering a July day across the UK takes about 290 W of solar, which fits on many van roofs. Covering a December day takes over 2,600 W, which doesn't. In winter, most of that has to come from a DC-DC charger while you drive, or from hook-up.
Roam's Build your electrics tool keeps this list for your van, shows the summer and winter picture side by side, and carries the results through to DC-DC, inverter and cable sizes with a live wiring diagram. It's in the app now, in early access.
Once you know the sizes, use the cable size calculator for every run and the inverter size guide if you're running 230 V. The whole process is on the campervan electrics page.
Questions
How much solar do I need on a campervan in the UK?
For summer, daily watt-hours ÷ about 4.8 peak sun hours ÷ 0.75. A typical 860 Wh day needs about 240 W in July. In winter, solar alone usually can't keep up, so plan on driving or hook-up too.
What size leisure battery do I need for 2 days off-grid?
Daily amp-hours × 2, divided by the usable share. For a 1,000 Wh day that's about 184 Ah of lithium or about 333 Ah of AGM.
Is lithium worth it over AGM?
You can use much more of a lithium battery's rating, so you need far fewer amp-hours for the same days off-grid, and it weighs less. It costs more up front. Check the battery's own datasheet for its usable depth and cycle life.
Does solar work on cloudy days in the UK?
Panels still produce something under cloud, but much less. The PVGIS averages already include typical UK cloud, but a run of grey days can fall well below them.