Solar Panel Sizing Guide for UK Van Life – How Many Do You Need?

How Many Watts of Solar Do You Need for UK Van Life?

Sizing a solar system is the single most common point of confusion in a UK van conversion. Buy too little and you are rationing power by February; buy too much and you are paying for roof space you will never use. The good news is that with a few honest numbers — your real daily consumption and the UK's genuinely modest peak sun hours — you can size a system that actually works in British weather rather than in a Southern European demo video.

The UK gets roughly 1,000–1,500 hours of sunshine per year depending on where you are, which is about half of southern Spain. Cloud cover knocks panel output down by 50–70% compared with clear-sky conditions. That means a UK system must be sized with far more margin than the same setup in the south of France or Portugal. This guide walks through the exact calculation, with worked examples you can copy straight into your own build.

Step 1: Work Out Your Real Daily Consumption

Do not guess. List every device, its wattage, and how many hours per day it actually runs. Multiply to get watt-hours (Wh) per day. Here is a realistic breakdown for a part-time UK van lifer:

A couple of notes on the numbers above. A compressor fridge is not on 24/7 — it cycles, so you count roughly half its rated hours. An inverter on standby still draws a few watts constantly, which adds up over a week. If you run a diesel heater's controls, a 12V heater, or a CPAP machine, add those in too — a CPAP alone is 40–60W for 8 hours (320–480Wh).

Step 2: Use Honest UK Peak Sun Hours

"Peak sun hours" is the number of hours per day a panel produces as if it were running at full rated output. The UK averages about 3 in good summer weather, but this swings hard by season and region:

Scotland and the north-west get less; the south-west (Cornwall, Devon, the Cotswolds) gets a little more. When you size a system, use the worst season you actually plan to use the van, not the sunny summer average.

The Sizing Formula

Minimum panel watts = daily consumption ÷ peak sun hours (worst realistic season), then add 20–30% headroom for battery age, wiring losses and dirty panels.

Worked example: 635Wh ÷ 2.5 (autumn) = 254W. Add 25% headroom = ~320W, so spec a 400W array (two 200W panels) to be safe. If you only use the van in the warmer months, 200W is enough for that 635Wh day.

Recommended Setups by Use Case

Weekend trips (lights, phone, no fridge):

• 100–150W panel, 50–100Ah battery
• Summer output 300–450Wh, winter 50–100Wh
• Cheapest route in; fine for a few nights away

Part-time van life (fridge, lights, laptop, pump):

• 200–400W array, 100–200Ah lithium
• Summer 600–1,500Wh, winter 150–300Wh
• The sweet spot for most UK van lifers

Full-time off-grid (fridge, inverter appliances, heater controls, CPAP):

• 400–600W+ array, 200–400Ah lithium
• Add a DC-DC charger for winter, or you will be running a generator
• Budget for EHU (shore) power at campsites too

Do Not Forget Battery Capacity

Panels only fill the bucket; the bucket is the battery. A common mistake is a big array on a small battery, which means you generate power you cannot store. A useful rule: battery capacity (Wh) ≈ 1.5–2× your daily consumption, so you survive a bad day without running flat.

For a 635Wh day, that is ~1,000–1,300Wh. At 12.8V (lithium), that is roughly 80–100Ah of LiFePO4. Lithium is worth it in the UK because you can safely use ~90% of capacity, it handles the colder-chemistry sag better, and it lasts 3–4× longer than AGM for the same cycle count.

Panel Types

Fixed rigid panels — most efficient, longest lasting, best all-weather performance. Mount with a stand-off gap under the panel for airflow; a hot panel loses a few percent of output. Around £0.80–1.20 per watt.

Flexible panels — light, low-profile, bond onto curved roofs. Less efficient, and the cell layer is more vulnerable to heat and delamination over years. Around £0.60–1.00 per watt.

Portable panels — brilliant supplement when you park in shade. Set them on the ground or a trolley facing the sun and top up in the afternoon. Around £1.00–1.50 per watt.

MPPT vs PWM Controller

An MPPT controller tracks the panel's maximum power point, so it recovers output when the battery is low or the sky is grey — exactly the UK's usual conditions. It is typically 20–30% more productive than PWM here, and it lets you run panels at a higher voltage than the battery (useful for series strings). Cost is £50–150.

A PWM controller is £15–30 and fine for a small 100W system in good sun, but in a cloudy UK climate the MPPT pays for itself within a year or two. For anything over 100W, get MPPT.

UK Winter Reality Check

This is the part most people skip. In December a 200W panel may only make 50–100Wh a day — enough for lights and a phone, not a fridge. If you use the van through winter you need either more panel (400W+), a DC-DC charger that recovers power while you drive, or regular EHU top-ups at campsites. Tilting panels toward the sun (30–40°) helps in winter; a flat roof loses roughly 20% of winter generation versus a tilted mount.

Sizing Mistakes to Avoid

• Sizing off a sunny summer day instead of the worst month you use the van.
• Big array, undersized battery — you generate power you cannot store.
• Ignoring inverter standby draw, which quietly eats a battery overnight.
• Spec'ing PWM for a large system in a cloudy climate.
• Forgetting headroom for battery age, so the system sags after two winters.

Quick Reference

Start with an honest consumption table, size for the worst realistic month, and add headroom. 200–400W with a lithium bank is the sweet spot for UK van life, and an MPPT controller is the one upgrade that is almost always worth it.

Wiring, Fusing and Breakers

Sizing the panels is only half the job. The wiring and protection between panel, controller and battery is where most UK conversions quietly go wrong. A few rules of thumb:

• Fuse at the battery first. Put the main fuse/breaker as close to the battery positive as you can (ideally under 30cm). This is what protects the whole circuit.
• Size the cable, not just the fuse. A 200W system at 12V draws around 13–16A from the panels into the controller; a 400W system around 25A. Use cable that is rated comfortably above that for your run length so you do not drop voltage or overheat the wires.
• Keep the run short. Every metre of cable is voltage loss. Where possible, mount the controller near the battery and keep the panel cable as short as the roof layout allows.
• Use MC4 or properly crimped connectors. Crimped or soldered-and-sleeved joints, not spade terminals that can back out.
• Ground the panels. A proper earth path protects both the system and the van.

Get this part right and the system will run for years without a fuse blowing or a hot connector.

Monitoring Your System

Once it is built, you want to know what it is actually doing — especially in a cloudy climate where you can guess wrong easily. A few options:

• A shunt-based battery monitor (e.g. a Victron SmartShunt or similar) tells you the battery's true state of charge and the charge/discharge currents, so you can see exactly how much the panels are adding each day.
• The controller's display shows instantaneous panel power; glance at it on a grey day to see how much you are really getting.
• A simple log for a month. Note the battery % each evening and the panel Wh each day across a range of weather. This is the fastest way to learn whether your sizing was right or whether you need more array, a DC-DC charger, or a bigger battery.

Monitoring is what turns "I hope it works in winter" into "I know it will."

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