12V campervan electrics calculator
Work out your power budget, battery size, solar and DC-DC charging month by month, and the right cable and fuse for each circuit. Free, no sign-up, nothing leaves your browser.
1 · Power budget
List what you run in a typical day. Duty cycle is the share of time a device actually draws power while on: a compressor fridge cycles on and off, so about 30–40% is typical. AC appliances are divided by inverter efficiency.
| Appliance | DC/AC | Watts | Qty | Hours/day | Duty % | Wh/day |
|---|
2 · Battery size
Usable capacity: LiFePO4 is typically used to about 80% depth of discharge, and lead-acid to about 50% for a reasonable lifespan. Check your battery maker's datasheet.
3 · Charging month by month
Table
| Month | Peak sun hours | Solar Wh/day | DC-DC + hook-up Wh/day | Balance Wh/day | Solar W to cover use alone |
|---|
Solar estimates use the EU Joint Research Centre's PVGIS (SARAH-3, 2020–2023 average) for a flat, roof-mounted panel. The system factor (75% by default) covers heat, dirt, controller and cable losses. Real-world output varies with shade, parking direction and weather.
4 · Cable size & fuse
Cable area = 0.0175 × 2 × length × current ÷ (voltage × allowed drop). The 2 × counts the positive and negative runs. The suggested fuse is the next standard size at or above 1.25 × the current, and the cable is upsized until its current rating (ISO 10133:2017, Table A.1) is at least the fuse rating. Fit the fuse within 175 mm of the battery or power source.
The 12V Van Electrics Guide (PDF)
18 pages: two wiring diagrams, three fully worked builds (weekender, remote worker, winter tourer), cable and fuse tables, and a pre-switch-on safety checklist. English and French editions in one download.
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Guides
FAQ
What size leisure battery do I need for a campervan?
Add up your daily energy use in watt-hours, multiply by the number of days you want to go without charging, then divide by the usable depth of discharge (about 0.8 for LiFePO4, 0.5 for lead-acid). Divide by 12 to get amp-hours. For example, 1,000 Wh/day for 2 days on LiFePO4 is 2,500 Wh, or about 208 Ah, so a 200–230 Ah lithium battery.
How much solar do I need on a van in the UK?
It depends heavily on the month. In London, a flat panel averages about 5.5 peak sun hours a day in June but only about 0.6 in December (PVGIS, 2020–2023). With 200 W and a 75% system factor, that's roughly 830 Wh a day in June against about 90 Wh in December. Most UK vans need a DC-DC charger or hook-up to get through winter.
What size cable do I need for 12V?
Use cable area (mm²) = 0.0175 × 2 × one-way length (m) × current (A) ÷ (12 × allowed drop). Allow 3% drop for critical loads such as a fridge, pump or charger, and 10% for lighting. Then check that the cable's current rating is above the fuse rating. The calculator does both.
What fuse should I use?
A common rule of thumb is the next standard fuse size at or above 1.25 × the circuit's normal current, and never higher than the cable's current rating. The fuse protects the cable, so fit it as close to the battery as possible (within 175 mm is a widely used standard).
Do I need a DC-DC charger or a split-charge relay?
Most modern vans with smart alternators (Euro 6) need a DC-DC charger because the alternator voltage varies. A DC-DC charger also charges lithium batteries properly. Check your van's handbook and the charger maker's guidance.
Is this calculator accurate?
It uses standard formulas and published data (PVGIS for sunshine, ISO 10133 for cable ratings), but it's a planning tool. Real use, weather, shading and installation quality all vary, so build in a margin and check your equipment's datasheets.
Safety & disclaimer
This calculator is a planning aid, not a substitute for a qualified electrician or the equipment manufacturer's instructions. Mains (230 V) work, including hook-up and inverter output wiring, should be installed or checked by a competent person. Always fuse every positive cable at the source. Figures are estimates.