Short answer: plan campervan electrics in this order. Add up what you'll run each day in watt-hours, size a leisure battery to cover your days off-grid (lithium gives you more of its rating than AGM), add solar knowing UK winters give a fraction of summer, charge from driving with a DC-DC charger, add an inverter only for 230 V kit, then size every cable and fuse.
When we planned the electrics in our own van, the mistake we nearly made was buying parts first. A battery on offer, a solar panel that looked about right. The parts only make sense once you know what you'll use, so this guide goes through campervan electrics in the order the decisions depend on each other. Each step has a tool or a deeper guide if you want to go further.
Campervan electrics tools and guides
1. Start with a power budget
Everything else is sized from one number: how many watt-hours (Wh) you use in a day. For each thing you'll power, multiply its watts by the hours it runs, and by how many you have. Some things cycle on and off. A compressor fridge's label might say 45 W, but the compressor only runs part of the time, so the daily figure is lower than 45 W times 24 hours. Roam's planner uses a duty cycle for this: the share of the time it's actually drawing power.
Anything that runs on 230 V goes through an inverter, and the inverter wastes some power doing that. Roam's maths assumes 90% efficiency by default, so a 1,000 W kettle used for 9 minutes (0.15 hours) costs about 150 Wh at the kettle but about 167 Wh from the battery.
Here's a typical weekend set-up using the starting figures in Roam's planner. These are typical numbers, not measurements of your kit, so check your own labels:
| Item | Figures | Wh a day |
|---|---|---|
| Compressor fridge | 45 W, 24 h, runs 35% of the time | 378 |
| LED lights | 6 × 3 W, 4 h | 72 |
| Water pump | 50 W, 0.3 h | 15 |
| USB chargers | 2 × 10 W, 2 h | 40 |
| Laptop charging | 65 W, 3 h | 195 |
| Roof fan (summer) | 20 W, 8 h | 160 |
| Total | 860 |
The solar and battery calculator does this sum for your own list and carries it through the next two steps.
2. Leisure battery: lithium or AGM
A battery's amp-hour (Ah) rating isn't all usable. How much you can take out without shortening its life is the depth of discharge. Victron's datasheet for its 12.8 V lithium iron phosphate (LiFePO4) batteries rates them at 2,500 cycles at 80% depth of discharge and 5,000 cycles at 50%. Lead acid types, AGM and gel, wear faster the deeper you take them; Victron's AGM and gel datasheet says repeated deep discharge has a very negative effect on service life. Roam's planner counts 85% of a lithium battery as usable and 50% of an AGM, a common rule of thumb rather than a law.
To size the battery, turn watt-hours into amp-hours (divide by 12.8 V for lithium, 12 V for lead acid), multiply by the days you want to go without charging, then divide by the usable share. For the 860 Wh day above and two days off-grid:
- Lithium: 860 ÷ 12.8 = 67 Ah a day, × 2 days ÷ 0.85 = about 158 Ah.
- AGM: 860 ÷ 12 = 72 Ah a day, × 2 days ÷ 0.5 = about 287 Ah.
That gap is why so many van builds now go lithium, even though it costs more up front: you need a much smaller, lighter battery for the same usable energy. Victron's datasheet also notes lead acid charging becomes very inefficient above about 80% charge, which matters when you rely on a short drive or a cloudy day to top up.
3. Solar across UK seasons
A solar panel's watt rating is what it makes in strong, direct sun. What you get in a day depends on how much sun there is, which in the UK changes enormously through the year. We pulled the monthly figures from PVGIS, the European Commission's Joint Research Centre solar tool, for flat panels (like most van roofs) in Cornwall, London, Birmingham, Manchester, Edinburgh and Inverness, averaged over 2011 to 2020.
| Month | UK average peak sun hours a day |
|---|---|
| December | 0.52 |
| March | 2.43 |
| June | 4.78 |
| July | 4.85 |
| September | 2.93 |
To find the panel watts you need: daily Wh ÷ peak sun hours ÷ 0.75. The 0.75 is the allowance Roam's planner uses for real-world losses (heat, dirt, wiring, the controller). For our 860 Wh day, July needs about 236 W of solar. December needs about 2,200 W, far more than fits on a van roof. That's normal. In the UK, solar can carry you through summer and does very little in midwinter, so winter vans lean on driving and hook-up instead. These are long-term averages: a grey week in July can be far worse.
On the solar side, a charge controller (MPPT or PWM) sits between the panels and the battery. Check the panels' open-circuit voltage against the controller's limit, remembering it rises in cold weather.
4. DC-DC charging from the engine
A DC-DC charger (also called a battery-to-battery or B2B charger) takes power from the vehicle's alternator and charges the leisure battery properly while you drive. It's the step that saves winter trips. Many newer vans have smart alternators, whose voltage varies. Victron's Orion XS manual says a smart alternator's voltage can vary between 12.5 V and 15 V on a 12 V system, and that Euro 6 vehicles can switch the alternator off for a while. A DC-DC charger is designed to cope with that, and the Orion XS lets you cap the current it draws so you don't overload the alternator.
The rough sum: daily Wh you need from driving ÷ (charger amps × 12.8 V) = hours of driving. A 30 A charger puts back about 384 Wh an hour. The full method, with alternator limits, is in the DC-DC charger size guide.
5. Inverter, inverter-charger and hook-up
Only fit an inverter if you need 230 V. Many van things (fridge, lights, pump, fan, USB, some laptop chargers) run on 12 V directly with no inverter losses. If you do want a kettle, air fryer or hair dryer, the inverter has to handle the biggest single 230 V thing you'll run, plus a margin for start-up surges. Continuous output also drops as the inverter gets warm: Victron's datasheet gives its Phoenix Inverter Smart 12/1600 as 1,300 W at 25 °C but 1,200 W at 40 °C.
The current on the battery side is the bit people miss. A 1,000 W load through a 90% efficient inverter at 12 V pulls about 93 A from the battery, which needs heavy cable and a big fuse. See the inverter size guide.
An inverter-charger combines an inverter with a mains battery charger and a transfer switch: when you plug into electric hook-up at a campsite, it passes mains through and charges the battery. Victron's MultiPlus-II 12/3000/120-32, for example, has a 32 A transfer switch and up to 120 A of battery charging. A simpler route is a separate mains charger fed from your hook-up inlet. Either way, the 230 V side is covered by the wiring regulations below.
6. Cables and fuses
Every cable needs two checks. It must be thick enough that the voltage lost along it stays small, and thick enough to safely carry the current of the fuse protecting it. Voltage drop gets worse with length, and on 12 V a fraction of a volt matters. Victron's Wiring Unlimited aims for under 2.5% on its own systems. Roam's cable tool uses 3% for anything that needs a steady voltage (motors, chargers, electronics) and allows up to 10% for lights and USB, which cope with a small dip.
Victron's guidance on fuses is simple: every consumer connected to a battery gets its own fuse, in the positive cable, however small it is, and the system has a main battery fuse before it branches off. Roam picks the next standard fuse at or above 1.25 times the load, then sizes the cable so that fuse protects it. Try it with the cable size calculator.
Roam's Build your electrics tool does all of this in one place: power budget, battery, solar, DC-DC, inverter sizing, a live wiring diagram and a shopping list of cable, fuses and lugs. It's in the app now, in early access.
7. Draw the wiring diagram
Before you buy cable, draw the whole system: battery, main fuse, isolator, busbars, each charger and each load, with the length of every run. The lengths go back into your cable sizes, and the drawing becomes the shopping list and the map you'll need when something stops working on a dark night in a lay-by. Our wiring diagram guide walks through a typical layout part by part.
8. When to use an electrician
Plenty of people wire their own 12 V system carefully and well. The 230 V side is different. BS 7671, the UK wiring regulations, has a section (721) specifically for caravans and motor caravans, covering things like RCDs, electrical separation and the hook-up connection, according to the IET's own summary. Mains in a moving, damp, metal box can kill, so we'd have a qualified electrician install or at least inspect and test anything on the 230 V side, including the hook-up inlet and consumer unit. Gas belongs with a Gas Safe registered engineer.
It's also worth getting help if your system is large (high current inverters, 24 V or 48 V), or if the van's own wiring or smart alternator setup is unusual. Our numbers here and in the tools are guidance, not a substitute for a qualified installer.
Questions
How much battery do I need in a campervan?
Add up your daily watt-hours, divide by 12.8 for lithium (12 for AGM), multiply by the days you want without charging, and divide by the usable share: about 0.85 for lithium and 0.5 for AGM on Roam's figures. A typical 860 Wh day for two days comes out at about 158 Ah of lithium.
Will solar keep my campervan battery charged in winter?
Usually not on its own. PVGIS figures for flat panels average about 0.5 peak sun hours a day across the UK in December against nearly 5 in July, so winter charging mostly comes from driving through a DC-DC charger, or from electric hook-up.
Do I need an inverter in a campervan?
Only for things that need 230 V. Lots of van kit runs straight off 12 V, which is more efficient. If you only charge phones and a laptop, a 12 V charger may be all you need.
Can I wire my own campervan electrics?
Many people do the 12 V side themselves. Anything 230 V, including hook-up, falls under BS 7671 Section 721, so have a qualified electrician install or test it.
What order should I do campervan electrics in?
Power budget, battery, solar, DC-DC charger, inverter, then cables and fuses, then the wiring diagram. Each step depends on the one before it.