Whether you're building out a campervan for weekend trips or full-time living, solar is one of the best investments you can make. But how much solar do you actually need? Too little and you'll be constantly running your engine to charge batteries. Too much and you've wasted money and roof space. This guide helps you calculate the right amount of solar for a UK-based campervan.
Step 1: Work Out Your Daily Energy Usage
Before you can size your solar, you need to know how much energy you use each day. List every 12V device in your van, its power consumption in watts, and roughly how many hours you use it per day.
Typical Campervan Daily Energy Budget
| Device | Power (W) | Hours/Day | Wh/Day |
|---|---|---|---|
| LED lighting (4 lights) | 20W total | 4h | 80 Wh |
| Compressor fridge (Dometic etc.) | 45W (avg) | 12h (cycles on/off) | 540 Wh |
| Phone + tablet charging | 15W | 3h | 45 Wh |
| Laptop charging | 60W | 2h | 120 Wh |
| Water pump | 60W | 0.25h | 15 Wh |
| Diesel heater fan | 30W (avg) | 8h (winter) | 240 Wh |
| USB fan | 5W | 6h (summer) | 30 Wh |
Total: approximately 830–1,070 Wh per day depending on season.
A typical UK campervan uses between 500 Wh and 1,200 Wh per day. Weekend warriors at the lower end, full-timers with laptops and diesel heaters at the upper end.
Step 2: Factor In UK Sunlight Hours
Solar panels don't produce their rated wattage all day. Output depends on sunlight intensity, angle, cloud cover, and temperature. In the UK, we measure usable solar energy in peak sun hours (PSH) — the equivalent number of hours at full rated power.
Average Peak Sun Hours by Season (Southern England)
| Month | Average PSH/Day |
|---|---|
| January | 1.0 |
| February | 1.5 |
| March | 2.5 |
| April | 3.5 |
| May | 4.5 |
| June | 5.0 |
| July | 4.8 |
| August | 4.2 |
| September | 3.0 |
| October | 2.0 |
| November | 1.2 |
| December | 0.8 |
Scotland gets roughly 10–20% less than these figures. Northern England is somewhere in between.
The key takeaway: UK summer gives you 4–5 PSH, but winter drops to just 0.8–1.5 PSH. If you want solar self-sufficiency in winter, you need significantly more panels than for summer-only use.
Step 3: Calculate Required Solar Wattage
The basic formula:
Solar watts needed = Daily Wh usage ÷ Peak sun hours ÷ System efficiency
System efficiency accounts for losses in the MPPT controller, cabling, and battery charge/discharge. A realistic figure is 0.80 (80% overall efficiency) for a well-designed system.
Example: 800 Wh/day Usage
| Season | PSH | Solar Watts Needed |
|---|---|---|
| Summer (4.5 PSH) | 4.5 | 800 ÷ 4.5 ÷ 0.8 = 222W |
| Spring/Autumn (2.5 PSH) | 2.5 | 800 ÷ 2.5 ÷ 0.8 = 400W |
| Winter (1.0 PSH) | 1.0 | 800 ÷ 1.0 ÷ 0.8 = 1,000W |
This illustrates the fundamental challenge of UK solar: you'd need 1,000W to be solar self-sufficient in December, but only 222W in June. Most people compromise — size for spring/autumn and accept that winter will require supplementary charging from the alternator or hookup.
Step 4: Choose a Practical Panel Configuration
Given that most campervans have roof space for 2–4 panels, here are the realistic options:
| Configuration | Total Watts | Roof Space Needed | Suitable Victron MPPT | Best For |
|---|---|---|---|---|
| 1 × 200W | 200W | ~1.3m × 0.7m | SmartSolar 100/20 | Weekend summer trips, low usage |
| 2 × 200W | 400W | ~2.6m × 0.7m | SmartSolar 100/30 | Most campervans — spring to autumn |
| 3 × 200W | 600W | ~3.9m × 0.7m | SmartSolar 150/45 | Full-timers, larger vans |
| 2 × 200W + 1 × 100W | 500W | Mixed | SmartSolar 150/35 | Filling remaining roof space |
Step 5: Pair with the Right Battery
Your battery bank needs to store enough energy to cover at least 1–2 days without sun (overcast days, driving days when panels are partially shaded by roof boxes, etc.).
Rule of thumb for lithium (LiFePO4): Battery capacity (Ah) should be at least 1.5× your daily usage in Ah.
For 800 Wh/day: 800 ÷ 12.8V = 62.5 Ah. Multiply by 1.5 = 94 Ah minimum. A 100Ah lithium battery is the standard choice. 200Ah gives you two full days of buffer.
Rule of thumb for lead-acid (AGM/GEL): Double the lithium figure because you should only discharge lead-acid to 50%. So 94 Ah × 2 = 188 Ah minimum. A 220Ah AGM battery is the nearest common size.
Our Recommendation for Most UK Campervans
The sweet spot for most UK campervan builds:
- Solar: 2 × 200W panels (400W total)
- MPPT: Victron SmartSolar 100/30
- Battery: 100–200Ah lithium (LiFePO4) or 200–300Ah AGM
- Supplementary charging: Victron Orion-Tr Smart DC-DC charger for alternator top-up while driving
This setup gives comfortable energy independence from April to September and only needs alternator top-ups during the darker months. Combined with a DC-DC charger, even winter trips are manageable with a couple of hours of driving per day.
What About Flexible vs Rigid Panels?
Rigid monocrystalline panels (mounted on brackets with an air gap underneath) last longer, run cooler, and perform better than flexible panels glued to the roof. Flexible panels are lighter and lower profile, but they run hotter (reducing output by 10–15%) and have shorter lifespans (3–5 years vs 20+ years for rigid).
If you have the roof height and mounting options, always choose rigid panels. The performance difference compounds over years of ownership.
Summary
For a typical UK campervan using 500–1,000 Wh per day, 400W of solar (2 × 200W panels) paired with a Victron SmartSolar 100/30 is the ideal starting point. This covers spring through autumn comfortably. Add a DC-DC charger for winter alternator charging, and you'll have a well-balanced system that keeps you off-grid with confidence.
