How much solar power will you actually generate in the UK? The brochure numbers (300W panel = 1,200Wh/day) assume Arizona sun. In the UK, you'll be lucky to get half that in summer and a fraction in winter.
This guide gives you a realistic calculator for UK solar yield based on your panel size, van location, and time of year. No made-up numbers.
The UK Solar Reality
The UK receives significantly less solar radiation than most of Europe. The key figure is peak sun hours (PSH) — the equivalent number of hours per day when solar irradiance is at the standard test condition of 1,000W/m².
| Location | Summer (June) | Equinox (Mar/Sep) | Winter (December) |
|---|---|---|---|
| Cornwall/Devon | 4.5-5.0h | 2.5-3.0h | 0.8-1.2h |
| Midlands | 4.0-4.5h | 2.0-2.5h | 0.5-0.8h |
| Northern England | 3.5-4.0h | 1.5-2.0h | 0.3-0.5h |
| Scotland | 3.0-3.5h | 1.0-1.5h | 0.2-0.4h |
These numbers already account for cloud cover (the UK's average cloud cover is 60-70%). They are real-world averages, not theoretical maximums.
The Formula
Daily yield (Wh) = Panel wattage × Peak sun hours × System efficiency
System efficiency accounts for:
- Wiring losses (3-5%)
- MPPT controller efficiency (95-98%)
- Panel temperature derating (panels lose efficiency above 25°C — about 0.4% per °C)
- Dust/dirt on panels (2-5%)
- Angle/tilt losses (panels on a van roof are flat — you lose about 10-15% vs optimal tilt)
Realistic system efficiency: 75-80%
Quick Calculation
| Panel Size | Summer (South) | Summer (Scotland) | Winter (South) | Winter (Scotland) |
|---|---|---|---|---|
| 100W | 340-380 Wh | 225-260 Wh | 60-95 Wh | 15-30 Wh |
| 200W | 680-760 Wh | 450-520 Wh | 120-190 Wh | 30-60 Wh |
| 300W | 1,020-1,140 Wh | 675-780 Wh | 180-285 Wh | 45-90 Wh |
| 400W | 1,360-1,520 Wh | 900-1,040 Wh | 240-380 Wh | 60-120 Wh |
| 500W | 1,700-1,900 Wh | 1,125-1,300 Wh | 300-475 Wh | 75-150 Wh |
What This Means for Your Van
Summer (April-September)
A 200W panel in southern England generates 680-760Wh per day. This is enough to run:
- 12V compressor fridge (15Ah = 180Wh)
- LED lighting (2Ah = 24Wh)
- Laptop charging (60Wh)
- Phone/device charging (20Wh)
- Water pump (5Wh)
- Total: ~290Wh — you have 390-470Wh of surplus
You can run indefinitely with 200W + 100Ah LiFePO4 in summer, even without driving.
Winter (November-February)
The same 200W panel in southern England generates 120-190Wh per day. This is enough to run:
- LED lighting (2Ah = 24Wh)
- Phone charging (10Wh)
- Water pump (5Wh)
- Total: ~39Wh — you have 81-151Wh of surplus
Your fridge (180Wh/day) alone exceeds the winter solar yield. In winter, a 200W panel cannot sustain a fridge without additional charging (driving or mains hook-up).
The Winter Gap
The gap between winter solar yield and your consumption determines whether you need to drive or plug in:
| Panel | Winter Yield (South) | Fridge Only | Fridge + Lights + Devices | Surplus/Deficit |
|---|---|---|---|---|
| 100W | 60-95 Wh | -85 to -120 Wh | -130 to -165 Wh | -65% to -75% deficit |
| 200W | 120-190 Wh | +10 to -60 Wh | -35 to -105 Wh | -25% to -55% deficit |
| 300W | 180-285 Wh | +100 to +105 Wh | +25 to +130 Wh | +15% to +75% surplus |
| 400W | 240-380 Wh | +160 to +200 Wh | +85 to +245 Wh | +50% to +160% surplus |
Takeaway: In winter in southern England, 300W is the minimum for indefinite off-grid living with a fridge. At 200W, you need to drive for 30-60 minutes every 2-3 days to keep the battery topped up. At 100W, you will run out of power within 48 hours.
Scotland in Winter
In northern Scotland (December, 0.2-0.4 PSH), a 300W panel generates only 45-90Wh per day. Even 400W only gives 60-120Wh. You cannot rely on solar alone in Scotland in winter. Plan for daily driving, a wind generator, or mains hook-up.
Panel Configuration
Flat Mount vs Tilt
Most van solar panels are mounted flat on the roof (aesthetic, aerodynamic, secure). Flat mounting reduces yield by 10-15% compared to a 30-45° tilt.
Tiltable mounts (e.g., Renogy tilt brackets, £25-40) let you angle your panels toward the sun. In winter, when the sun is low in the sky (15-20° elevation at noon in December), tilting panels can increase yield by 30-50%.
The catch: You need to be parked to tilt them, they catch more wind (risk of damage in storms), and they look less stealthy.
Portable Panels
A 100-200W portable panel (foldable, placed on the ground facing the sun) can supplement your roof panels significantly. In winter, when the sun path is low, a portable panel angled at 60° can produce 2-3x more than the same wattage flat on the roof.
Best portable panels for UK vans:
- Renogy 100W Foldable (£150-180)
- BigBlue 100W Solar Charger (£100-130)
- Eco-Worthy 120W Portable (£80-110)
Battery Sizing for UK Solar
Your battery needs to bridge the gap between generation and consumption over cloudy days.
Rule of thumb: In the UK, you should size your battery to cover 3-5 days of consumption without any solar input (the "winter grey spell").
| Daily Consumption | Battery Size (LiFePO4) | Days of Autonomy |
|---|---|---|
| 300 Wh (light use) | 100Ah | 3-4 days |
| 500 Wh (fridge + basic) | 150-200Ah | 3-4 days |
| 800 Wh (fridge + laptop + lights) | 200-300Ah | 3-5 days |
LiFePO4 batteries are essential for effective solar in the UK. Lead-acid/AGM batteries can only use 50% of their rated capacity and charge less efficiently (PWM controller with AGM loses a further 20-30%).
Practical UK Solar Calculator
Simplified formula for van lifers:
Daily yield = Panel watts × PSH × 0.78 (efficiency)
| Your Panel | Formula | Summer (South Wh) | Winter (South Wh) |
|---|---|---|---|
| 100W | 100 × PSH × 0.78 | 100 × 4.5 × 0.78 = 351Wh | 100 × 1.0 × 0.78 = 78Wh |
| 200W | 200 × PSH × 0.78 | 200 × 4.5 × 0.78 = 702Wh | 200 × 1.0 × 0.78 = 156Wh |
| 300W | 300 × PSH × 0.78 | 300 × 4.5 × 0.78 = 1,053Wh | 300 × 1.0 × 0.78 = 234Wh |
| 400W | 400 × PSH × 0.78 | 400 × 4.5 × 0.78 = 1,404Wh | 400 × 1.0 × 0.78 = 312Wh |
Plug in your panel size and your location's PSH (from the table at the top) to get your realistic yield.
