Size your campervan battery & solar honestly
Campervan, boat, cabin or off-grid build — tick what you run, and get a realistic lithium battery size and solar panel wattage for UK conditions. No sales inflation, no guesswork.
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Indicative sizing at 12V with 15% system losses, lithium ~90% / AGM ~50% usable capacity, and UK peak-sun averages (~4.2h summer, ~2.5h spring/autumn). UK winter sun is closer to 1 hour — plan on hook-up or charging from the engine (B2B) if you’re out in winter. Every setup is different; we’ll size yours properly before anything is fitted.
Sized. Supplied. Fitted. Monitored.
We design and install Victron-based off-grid power — lithium, solar, B2B charging and remote monitoring — for boats, homes and off-grid buildings across Dorset. Our own support van runs on it (watch it live).
Also: Our Off-Grid Work
// GOOD QUESTIONS
Frequently asked
How accurate is this calculator?
It’s honest, indicative sizing: your devices’ watts × hours, plus 15% real-world system losses, sized against lithium (~90% usable) or AGM (~50% usable) and UK average peak-sun hours. Real installs vary with panel angle, shading, battery age and weather — which is why we always survey before we fit anything.
Why is winter solar so different?
UK peak-sun drops from roughly 4 hours a day in summer to under 1 in December — a sixth of the harvest or less. Winter setups usually pair solar with charging from the engine (a B2B charger) or hook-up, rather than fitting an enormous array.
Lithium or AGM leisure battery?
Lithium costs more upfront but you can use ~90% of its capacity, it charges faster, lasts many times longer and weighs half as much. AGM is cheaper but you should only use ~50% of the label capacity. That’s why the AGM equivalent in the results is nearly double the lithium size.
What kit do you install?
We design and fit Victron-based systems — lithium batteries, MPPT solar controllers, B2B chargers, inverters and remote monitoring — for campervans, boats and off-grid buildings. Our own support van runs on one; you can watch its live dashboard.
Can you check my numbers?
Gladly — tell us what you run and how you travel, and we’ll size it properly (and tell you honestly if a smaller system will do). Get in touch.
// FREE TOOLS
More free tools for life on the road
No sign-up, no catch — free tools built by our techies. See all our free tools →
Free tool
Victron System Builder
Four questions → a complete, honestly-sized Victron kit list with links to buy every part.
Try it →Free tool
Live Broadband Speed Test
Measure your real download, upload and ping right now on a live animated gauge.
Try it →Free tool
Wi-Fi QR Code Generator
Make a QR code guests scan to join your Wi-Fi — no typing the password.
Try it →Powered by the people who live it
Our own mobile support van runs on the Victron kit we install — solar, lithium and remote monitoring, streaming live on our website. We’ll design yours the same way: properly.
01202 775566 · help@365techies.co.uk · MON–FRI 9AM–5PM
// HOW IT WORKS
How to size a campervan leisure battery and solar array
This free UK calculator sizes your leisure battery and solar panels by adding up the daily watt-hours of the appliances you tick, adding 15% for real-world losses, then giving an honest lithium (or AGM) battery capacity in Ah and the solar wattage needed for UK summer and winter sun.
Step by step
- Tick what you run — Check each appliance you use — fridge, lights, phone charging, laptop, diesel heater, CPAP and more — from the preset list.
- Set the daily hours — Adjust the hours-per-day box next to each ticked item so it matches how you actually use it; add any custom device by watts and hours.
- Choose your buffer — Slide ‘Days of autonomy’ (1–3) to set how many cloudy days you want the battery to cover without charging.
- Read the battery size — See your daily draw in Ah and the recommended lithium battery size (and the larger AGM equivalent), matched to common 100–200Ah blocks.
- Read the solar wattage — Check the summer and shoulder-season panel wattage; remember UK winter sun is near 1 hour a day, so plan on engine (B2B) or hook-up charging then.
Good to know
What size leisure battery and solar do I need for a campervan?
Add up the watt-hours of everything you run in a day, add about 15% for losses, then divide by 12 for amp-hours per day; multiply by your cloudy-day buffer and divide by 0.9 for a lithium battery size. This calculator does that automatically and also gives the panel wattage for UK summer and shoulder-season sun.
How many watts of solar do I need for UK conditions?
The tool sizes panels from your daily use against roughly 4.2 peak-sun hours in summer and about 2.5 in spring and autumn, plus a headroom margin. UK winter gives closer to one sun-hour a day, so it flags that solar alone won’t cover winter — pair it with engine (B2B) charging or hook-up.
Can I use this calculator for a boat, cabin or off-grid build?
Yes. It works for any 12V setup — campervan, boat, cabin or off-grid building — because it sizes from the appliances you actually run rather than a vehicle type. It gives indicative sizing at 12V; we’ll size yours properly, including cabling and fusing, before anything is fitted.