What size solar panel do you need for a campervan or motorhome in the UK?

Solar on a campervan or motorhome tops up a leisure battery that runs the fridge, lights and chargers, so you can stay off campsite hook-ups for longer. The hard part is the size: a flat roof panel in a Highland December gives less than a twentieth of what it gives in a southern July. This guide is for anyone fitting or buying a van with solar in the UK, including people who have never wired anything.

The short version

• Start with your daily use in watt-hours. A 12 V compressor fridge is usually the biggest steady load; a diesel heater fan matters in the cold months.
• On average, 100 W of panel lying flat on a roof gave about 400 Wh a day in July in London, but about 46 Wh a day in December (PVGIS records for 2005 to 2023). In Inverness the December figure was about 14 Wh.
• Flat panels lose most in winter. Tilting the same panel to 35 degrees more than doubled the London December figure.
• A UK van roof cannot carry enough solar for a Scottish winter. In the dark months, plan on driving, hook-up or both.
• Modern vans may have a "smart" alternator that the vehicle's computer controls. A DC-DC charger lets it charge a leisure battery properly.
• 230 V wiring inside a van is a job for a qualified electrician and follows BS 7671 Section 721.
• Our quiz sizes for the weakest month of your trips, not the average one.

Step 1: work out what you use each day

Two units matter. Watts (W) are how fast a device uses power, like how far a tap is open. Watt-hours (Wh) are how much energy it uses over time, like how much water ends up in the bucket. A 60 W device running for 2 hours uses 120 Wh. The glossary explains every term.

The usual leisure loads in a van:

• 12 V compressor fridge. The big one, because it runs day and night. One major manufacturer lists a 74-litre two-compartment model at 1.43 amp-hours per hour when it is 32 °C outside and 4 °C inside. An amp-hour (Ah) is a battery's way of counting charge; at 12 V, 1 Ah is about 12 Wh. So on a hot day that fridge uses about 34 Ah, roughly 410 Wh (our calculation). Smaller fridges and cooler weather usually mean less.
• Diesel heater. The fuel makes the heat, but the fan, pump and controls run on the battery. One leading heater maker's datasheet gives 13 to 30 W for its 2 kW diesel air heater. Over a 6-hour evening at the top of that range, that is 180 Wh (our calculation). Start-up draws more for a short time; your manual gives the figure.
• Lights, water pump, phones and a laptop. Small but they add up. The label or charger shows watts, or volts and amps (multiply them to get watts).

If you are not sure, use the higher figure.

Step 2: understand the 12 V system

The living area of a van runs on 12 V direct current (DC), the same kind of electricity a car uses. A leisure battery stores the energy, separate from the starter battery so you never flatten the one that starts the engine. Three things charge it: a solar charge controller between the panels and the battery (our guide to MPPT and PWM controllers explains the types), a DC-DC charger from the alternator while you drive, and a mains charger on hook-up. An optional inverter turns 12 V into 230 V for mains devices, but kettles and hair dryers drain a battery fast.

The 12 V installation in the living area of a motorhome is covered by the European standard BS EN 1648-2; the 230 V side is covered separately (Step 7).

Step 3: choose the leisure battery

Lead-acid (AGM or gel). Cheaper, but only part of the stored energy is usable if you want it to last. One major maker rates its AGM batteries at 600 cycles if you discharge them by half each time, and 400 cycles if you discharge them by 80 per cent. (A cycle is one discharge and recharge.) So a 100 Ah AGM battery gives you about 600 Wh per night if you stop at half (our calculation: 12 V × 100 Ah × 0.5).

Lithium iron phosphate (LiFePO4). Lighter, and you can use more of it. The same maker rates its LiFePO4 batteries at 2,500 cycles at 80 per cent discharge. A 100 Ah, 12.8 V LiFePO4 battery used to 80 per cent gives about 1,020 Wh (our calculation: 12.8 V × 100 Ah × 0.8).

LiFePO4 in the cold. They can run things in frost, but charging them when too cold damages them. That maker allows charging from +5 °C to +50 °C, while discharge is allowed down to -20 °C. Other makers may set the floor at 0 °C, so check the datasheet of yours. A van in a Scottish January easily drops below both, so keep the battery inside the heated living area, not in an outside locker, or choose one with built-in heating. Our guide to LiFePO4 batteries in cold weather goes into detail.

Step 4: know what a roof panel really gives in the UK

PVGIS, the European Commission's free solar calculator, uses satellite records of sunshine and weather from 2005 to 2023 for the UK. We asked it what 100 W of panel gives on an average day, with its default 14 per cent allowance for losses in cables, dirt and electronics, for London (southern England) and Inverness (Scotland), the points our quiz uses for those regions.

A panel lying flat on the roof, average energy per day for each 100 W of panel:

• London: about 419 Wh in June, 402 Wh in July, 145 Wh in October, 46 Wh in December.
• Inverness: about 360 Wh in June, 342 Wh in July, 98 Wh in October, 14 Wh in December.

The same panel tilted at 35 degrees and facing south:

• London: about 397 Wh in July and 117 Wh in December.
• Inverness: about 345 Wh in July and 46 Wh in December.

Summer is easy almost everywhere. In June and July, Inverness gets about 85 per cent of London's figure (our calculation).

Flat panels lose most in winter. In July, flat and tilted panels give almost the same. In December, a flat panel in London gives only about 40 per cent of what a tilted one gives, and over the whole year about 17 per cent less (our calculation: 84.5 kWh against 101.9 kWh a year per 100 W). The low winter sun hits a flat panel at a shallow angle; a folding panel propped up facing the sun helps at camp.

These are averages; one grey week can give far less. That is why we simulate every day of the record (Step 8).

Step 5: charge from the alternator with a DC-DC charger

While the engine runs, the alternator charges the starter battery, and in the older, simpler setup a split-charge relay joins the leisure battery to it. Many modern vans have a "smart" alternator that the vehicle's computer controls. One charger maker's manual explains: a conventional alternator gives a fixed voltage of about 14 V, while a smart one varies between roughly 12.5 V and 15 V, and vehicles with regenerative braking (recovering energy when you slow down) show large voltage swings. In Euro 6 vehicles the management can switch the alternator off for a while. A simple relay passes on whatever voltage is there, so the leisure battery may never fill.

A DC-DC charger (sometimes called battery-to-battery or B2B) fixes this. It converts whatever the alternator gives to the right charging voltage for your battery type and limits the current it draws. The same manual says that current limit is "particularly important to avoid overloading an alternator".

How much can driving give? A 30 A charger at around 14 V delivers about 420 W, so an hour of driving puts back a little under 420 Wh, because some energy is lost as heat in the charger (our calculation).

Chargers detect the running engine in different ways, so check the charger manual and your van maker's guidance before connecting to the vehicle's wiring.

Step 6: electric hook-up

A campsite hook-up feeds 230 V mains to your van's charger and sockets. The Camping and Caravanning Club says most of its sites can provide 16 A, some only 10 A, and other campsites as little as 5 A. The Caravan and Motorhome Club notes that a 1 kW load needs about 5 A at 230 V, and that a domestic kettle of more than 2 kW may not work on a 10 A outlet. It also recommends testing the van's RCD (residual current device, the safety switch that cuts power if current leaks to earth) every time you connect. For sizing, hook-up is your safety net: a hook-up every few nights in winter means far less solar.

Step 7: fuses, cables and what needs a professional

Fuses. In a battery system, every device needs its own fuse in the positive cable, and there is usually a main fuse near the battery. A battery can push a huge current into a short circuit. A charger maker's wiring guide says the fuse must not be rated higher than the cable it protects, must be rated for DC at your system voltage, and for lithium batteries must be able to break a very high fault current (its "interrupt rating"). It also says a fuse should sit between the solar panels and the charge controller.

Cable size. Thin cable wastes energy and can overheat. The same guide advises keeping the voltage drop to no more than 2.5 per cent, which is 0.3 V at 12 V. Our guide to solar cable size and fuses shows how to work it out.

What needs a professional:

• 230 V wiring inside the van. The UK wiring rules, BS 7671 (currently the 18th Edition with Amendment 4 of 2026), have a special section, 721, for caravans and motor caravans. It requires, for example, that each supply inlet connects directly to its RCD and that a main switch disconnects all live conductors. Adding sockets, a mains charger or an inverter feeding the van's mains circuits is work for a qualified electrician who knows this section.
• Habitation checks. The Approved Workshop Scheme, run by the National Caravan Council with the two big caravan clubs, says leisure vehicles should be serviced every year. Its FAQ adds that the IET recommends inspecting and testing the 230 V installation at least every 3 years, and that a damp test at each annual service keeps the body warranty. Roof holes for cables can let water in, so tell your workshop what you fitted.
• Warranty and insurance. Before drilling, check what your warranty and insurer say about added electrics.

Step 8: how our quiz sizes for your trips

In our quiz, choose "campervan / motorhome", pick up to three UK regions and the months you will be in each, then what you will power. Optional questions cover shade, flat or folding panels, daily driving hours, hook-up or a generator, and roof space.

We run your daily use through every day of many years of real weather records for each region and month, and size by the weakest link: the month and place that need the most. If you spend July in Devon and October in the Highlands, October in the Highlands decides the size. The result shows what drives the size, and three options with different reliability, built from parts whose limits we checked against manufacturer documents. Our methodology explains the method and its sources.

A worked example

Our calculation from the figures above, for an example van; your loads will differ. The fridge uses the hot-day figure, which is cautious for the UK:

• Fridge: 410 Wh
• LED lights, 20 W for 4 hours: 80 Wh
• Water pump, 60 W for 15 minutes: 15 Wh
• Two phones, 20 W for 2 hours: 40 Wh
• Laptop, 60 W for 2 hours: 120 Wh
• Diesel heater in October, 30 W for 6 hours: 180 Wh

Summer in southern England: 665 Wh a day without the heater. At 402 Wh per 100 W of flat panel in a London July, an average day needs about 165 W of panel (665 ÷ 402 × 100). Real days vary, so allow more, and a 100 Ah LiFePO4 battery (about 1,020 Wh usable) covers a dull day or so.

October in Scotland: 845 Wh a day with the heater. At 98 Wh per 100 W of flat panel in an Inverness October, an average day would need about 860 W of panel (845 ÷ 98 × 100). That is more than most van roofs can carry. With 400 W on the roof, the average October day gives about 390 Wh, leaving about 455 Wh to find. That is a bit over an hour of driving with a 30 A DC-DC charger, or a hook-up every two days.

December in Scotland: at 14 Wh per 100 W, solar is close to nothing on an average day. Plan on driving and hook-up.

What can go wrong

• Sizing for July, touring in October. The battery never fills and the fridge stops.
• A LiFePO4 battery in an outside locker. On a frosty morning it refuses to charge, or, without protection, it charges and is damaged.
• A relay on a smart alternator. You drive all day and the leisure battery is still half empty.
• A DC-DC charger too big for the alternator. Too much current can overload an alternator; set or choose a charger with a sensible input limit.
• Panel voltage too high for the controller. Panel voltage rises in the cold, so a controller rated close to the summer voltage can be damaged on a frosty morning.
• Thin cables, missing fuses. Cables get hot and a short circuit has nothing to stop it.
• A roof leak around a cable entry. Damp spreads unseen in the walls.

Questions people ask

Can I just plug a portable power station into the cigarette lighter? You can, but slowly. A charger maker's wiring guide says these car plugs cannot carry more than 10 A, and the van's circuit may be fused even lower, so they suit small loads only. Our guide to power stations and DIY systems compares the two.

Can I fit it myself? 12 V work needs care with fuses and cable sizes. Anything touching the 230 V system is a job for a qualified electrician, and your habitation check should note what you added.

Next step

Tell the quiz where and when you travel and what you power, and you will get three checked options sized for the weakest month of your trips, with every number explained. Read our methodology for how we calculate, and how big a solar battery you need for battery sizing.

Our results and guides are estimates and do not replace advice from a qualified electrician.