The electrical system is the nervous system of your campervan. Get it right and you have unlimited off-grid power. Get it wrong and you have dead batteries, blown fuses, or a fire.
I have wired three vans over the years. The first used a single leisure battery and a basic split-charge relay — it worked but ran out of power every other day. The second used a 200Ah LiFePO4 battery, 200W solar, and a DC-DC charger — that ran indefinitely in summer and needed a weekly drive in winter. The third (current build) uses 300Ah LiFePO4, 400W solar, a Victron inverter, and automated battery management — it handles everything including a laptop, fridge, lights, and a diesel heater for 3–4 days without sun.
This guide walks you through designing and wiring a UK campervan electrical system from scratch.
System Design: The Decision Tree
Before you buy anything, work out your total power consumption.
Step 1: Calculate your daily power budget
List everything you will run and estimate hours of use per day.
| Appliance | Power (W) | Hours/Day | Daily Consumption (Wh) |
|---|---|---|---|
| LED lights (4 × 3W) | 12W | 4h | 48Wh |
| 12V fridge | 5W (compressor running 30%) | 24h × 30% = 7.2h | 36Wh |
| Diesel heater | 10W (fan running 50% duty) | 24h × 50% = 12h | 120Wh |
| Laptop charging | 60W | 3h | 180Wh |
| Phone charging (×2) | 10W | 3h | 30Wh |
| Water pump | 30W | 0.5h | 15Wh |
| Total | 429Wh/day |
Add 20% margin for unexpected usage: 515Wh/day.
Step 2: Size your battery
Rule of thumb: battery capacity (Ah) = daily consumption (Wh) / battery voltage (12V) × days of autonomy.
For 2 days autonomy (typical UK winter where you might not drive for 48 hours):
- 515Wh × 2 days = 1,030Wh
- 1,030Wh / 12V = 86Ah
- Add 30% headroom for battery health: 110Ah minimum
For a full-time van lifer through a British winter with no hook-up: aim for 200Ah LiFePO4 minimum.
Step 3: Size your solar
UK solar output varies massively by season:
- June: ~4.5 sun hours/day
- December: ~0.8 sun hours/day
In winter, 400W of solar panels will produce ~320Wh/day in December (400W × 0.8h × 0.85 efficiency loss). That covers about 60% of the 515Wh/day demand. The rest comes from driving or hook-up.
| Panel Wattage | Summer (Wh/day) | Winter (Wh/day) |
|---|---|---|
| 200W | 765Wh | 136Wh |
| 300W | 1,148Wh | 204Wh |
| 400W | 1,530Wh | 272Wh |
| 500W | 1,913Wh | 340Wh |
Step 4: Size your inverter
An inverter converts 12V DC battery power to 240V AC for mains appliances.
| Appliance | Power Draw | Recommended Inverter |
|---|---|---|
| Laptop charger only | 60–90W | 300W pure sine wave |
| Laptop + TV | 150W | 500W pure sine wave |
| Microwave | 800–1,000W | 1,500W pure sine wave |
| Hair dryer | 1,500–2,000W | 2,500W pure sine wave |
Pure sine wave inverters are essential for sensitive electronics (laptops, CPAP machines, battery chargers). Modified sine wave inverters work for basic resistive loads (kettle, toaster) but cause buzzing in audio equipment and can damage some power supplies.
| Inverter Size | Typical Cost (Pure Sine) | Max 240V Current | Typical 12V Draw at Full Load |
|---|---|---|---|
| 300W | £50–80 | 1.3A | 30A |
| 500W | £80–120 | 2.2A | 50A |
| 1,000W | £120–180 | 4.3A | 100A |
| 1,500W | £180–250 | 6.5A | 150A |
Components
Battery
| Type | Capacity | Cost | Cycle Life | Weight | Best For |
|---|---|---|---|---|---|
| Lead-acid (AGM) | 100Ah | £80–120 | 400 cycles | 26kg | Budget builds, occasional use |
| LiFePO4 | 100Ah | £250–400 | 3,000+ cycles | 12kg | Full-time van life |
| LiFePO4 (high-end) | 200Ah | £500–800 | 5,000+ cycles | 22kg | Heavy users, winter UK |
LiFePO4 is the only sensible choice for full-time van life. Yes, it costs 3× more than AGM, but it lasts 8× longer, weighs half as much, and gives useful capacity down to -20°C (with internal heating). Over 5 years, LiFePO4 works out cheaper per Ah.
Recommended brands in the UK:
- Fogstar: Best value, UK-based, Bluetooth BMS on all batteries
- Renogy: Solid mid-range, good availability on Amazon UK
- Victron: Premium, very expensive, excellent build quality and support
- Eco-Worthy: Budget option, avoid for full-time use
Solar Charge Controller
| Type | Efficiency | Cost | Best For |
|---|---|---|---|
| PWM | 70–75% | £10–30 | Small panels (under 100W) |
| MPPT | 95–98% | £40–150 | All systems above 100W |
MPPT (Maximum Power Point Tracking) controllers convert excess voltage into additional current. In real terms, an MPPT controller gives 20–30% more charge than PWM in UK winter conditions. The extra cost is justified in one season.
Popular MPPT controllers:
- Victron SmartSolar 100/30: £130 — industry standard, Bluetooth
- Renogy Rover 40A: £90 — good mid-range
- EPEver Tracer 40A: £70 — budget but reliable
Split Charging (Charging from the Alternator)
When you drive, the van's alternator charges both the starter battery and your leisure battery. There are three methods:
| Method | Cost | Efficiency | Best For |
|---|---|---|---|
| Simple relay (VSR) | £15–30 | Moderate | Basic AGM systems |
| DC-DC charger | £80–200 | High | All LiFePO4 systems |
| Battery-to-battery (B2B) | £150–300 | Highest | High-power systems >50A |
DC-DC charger is the standard for UK vans. It takes the alternator's 14.4V output and converts it to the correct charging profile for your leisure battery (14.2–14.6V for LiFePO4). It also prevents the leisure battery from draining your starter battery.
Wiring a DC-DC charger:
- Run 16mm² (or 25mm² for 50A+) from starter battery to DC-DC charger in the van
- Run 16mm² from DC-DC charger output to leisure battery positive
- Connect chassis ground from DC-DC charger to a clean chassis bolt
- Wire the ignition trigger (D+ terminal) to an ignition-switched fuse in the cab fusebox
- Fuse both positive cables within 30cm of each battery
Fuse Box (Consumer Unit)
The 12V distribution board. Everything except the inverter and DC-DC charger runs through this.
| Model | Ways | Cost | Features |
|---|---|---|---|
| Blade fuse box | 6-way | £10 | Basic, no fuses included |
| Busbar with fuses | 8-way | £20 | Integrated busbar, clean install |
| Mains RCD + MCB | 6-way | £50 | For systems also running 240V |
Wire Sizing (Critical)
Undersized wire causes voltage drop, which causes poor charging, dim lights, and potentially overheated cables.
Wire Size Chart (for runs up to 5m total, positive + negative)
| Current (A) | Wire Size | Typical Application |
|---|---|---|
| 10A | 2.5mm² (14 AWG) | Lights, USB, diesel heater |
| 20A | 4mm² (12 AWG) | Water pump, small inverter |
| 30A | 6mm² (10 AWG) | Fridge, DC-DC charger input |
| 50A | 16mm² (6 AWG) | DC-DC charger output, main battery positive |
| 100A+ | 25mm² (4 AWG) | Inverter, battery bank main cable |
Colour Coding (UK Standard)
- Red: 12V positive (switched or permanent)
- Black: 12V negative (ground)
- Brown: 240V live
- Blue: 240V neutral
- Green/yellow: 240V earth
Step-by-Step Wiring Sequence
Phase 1: Battery and Main Cables
- Mount the leisure battery in a ventilated, accessible location (under seat or in a seat base box)
- Run 25mm² red cable from battery positive through a master fuse (250A for LiFePO4) to a distribution busbar
- Run 25mm² black cable from battery negative to a chassis ground point (clean paint, star washer, 8mm bolt)
- Fit a battery master switch on the positive cable (isolates the whole system)
Phase 2: Solar Panels
- Mount panels on roof with aluminium brackets and VHB tape (no roof drilling)
- Route MC4 cables through a solar entry gland in the roof
- Wire panels in series (for MPPT controller) or parallel (for PWM)
- Connect to solar charge controller
- Connect charge controller to battery busbar (fused at 20A)
Phase 3: DC-DC Charger
- Run 16mm² from starter battery + through a 50A MEGA fuse → DC-DC charger input
- Run 16mm² from DC-DC charger output → leisure battery +
- Connect ignition trigger (D+) to cab fusebox
- Test: start engine, confirm DC-DC shows charging (14.2–14.6V output)
Phase 4: Fuse Box
- Mount fuse box in a cabinet or under a seat
- Run 6mm² from battery busbar to fuse box input (fused at 30A)
- Connect a 6mm² ground wire from fuse box ground bus to chassis
- Wire each circuit: lights (5A), fridge (10A), water pump (5A), diesel heater (10A), USB sockets (10A)
Phase 5: Inverter
- Mount inverter within 1m of the battery (high current needs short cable runs)
- Run 25mm² from battery busbar to inverter positive (fused at 100A–200A depending on inverter size)
- Run 25mm² from battery busbar to inverter negative
- Wire inverter to a 240V socket with an RCD (if you plan to plug appliances in)
- Test: plug in a 100W lamp, confirm inverter works, check for overheating
Phase 6: Monitoring
A battery monitor is essential. It tells you state of charge, voltage, current draw, and remaining capacity.
- Victron SmartShunt: £90 — Bluetooth, excellent app, industry standard
- Renogy BT-2: £40 — works with Renogy batteries only
- BMV-712: £180 — premium, includes display panel in addition to Bluetooth
Common Mistakes
- Undersized cables: The most common error. Use the wire size chart above.
- No fuse at the battery: Every positive cable leaving the battery must have a fuse within 30cm. A short from an unfused cable can melt your entire wiring loom.
- LiFePO4 with a VSR relay: The simple split-charge relay does not have the correct charging profile for LiFePO4. You will severely undercharge the battery. Use a DC-DC charger.
- Mixing battery chemistries: Never connect a lead-acid starter battery in parallel with a LiFePO4 leisure battery through a VSR. Use a DC-DC charger.
- Inverter too far from battery: A 1,500W inverter at 12V draws 125A. Over 2m of 25mm² cable, that loses 0.5V — 4% of your voltage. Keep the inverter within 1m of the battery.
Maintenance
- Monthly: Check battery voltage at rest (13.3–13.4V for LiFePO4 at 100% SOC). Check all fuse holders for corrosion.
- Quarterly: Clean solar panels. Check wire connections for looseness. Torque battery terminals to spec.
- Yearly: Inspect all cables for chafing where they pass through holes. Replace battery monitor shunt battery.
FAQ
Q: Can I wire my van electrical system myself? A: Yes, with careful planning and proper research. The 12V side of a campervan electrical system is low-voltage DC, which is not regulated. The 240V AC side (inverter output or mains hook-up) requires either a professional electrician or Part P certification if integrated into the van permanently.
Q: How much does a full campervan electrical system cost? A: A full system including 200Ah LiFePO4 battery, 200W solar, DC-DC charger, MPPT controller, fuse box, and inverter costs £800–1,500 for DIY. Professional installation adds £500–1,000.
Q: Do I need an electrician to install a campervan electrical system? A: For the 12V side, no. For any mains voltage (240V) wiring — hook-up inlet, RCD, 240V sockets — you should use a qualified electrician.
Q: Can I run a microwave off my campervan battery? A: Possibly. A 1,000W microwave draws 83A from a 12V battery. You need a 1,500W inverter and at least 200Ah of LiFePO4 battery. At full load, you will drain 10% of your battery per minute of use. Realistic for occasional use, not daily.
Q: What size cable for a 50A DC-DC charger? A: 16mm² cable for runs under 5m. 25mm² for longer runs. Always fuse both positive ends.
Q: How long does a LiFePO4 battery last in a campervan? A: 3,000–5,000 charge cycles. If you cycle it once per day (full discharge and recharge), that is 8–14 years. In real van life use where you cycle 20–50% per day, it will outlast the van.
Q: Should I wire solar panels in series or parallel? A: Series, for an MPPT controller. Higher voltage means lower current means smaller cables and less voltage drop. For a 12V system, two 200W panels in series gives 40V open-circuit, which the MPPT converts to 14.4V at 26A. Parallel is only needed if shading is a major concern (one panel in shade drags the whole series string down).
