The formula
What are peak sun hours?
Peak sun hours are the number of hours per day the sun would need to shine at full strength (1,000 W/m²) to deliver the day's total energy. A summer day in the UK might have 15 hours of daylight but only 4–5 peak sun hours. In winter it can drop below 1. The best free sources for your exact location are the EU's PVGIS tool and NREL's PVWatts for the US.
Worked example
A van uses 1,200 Wh a day. In summer with 4 peak sun hours and 75% efficiency: 1,200 ÷ (4 × 0.75) = 400 W of solar, so two 200 W panels. In a UK winter with 1 sun hour you'd need 1,600 W, which won't fit on a van roof. That's why winter van-lifers rely on a DC-DC charger or hook-up.
Tips
- Size for the season you'll use the system most, not the yearly average.
- Flat panels on a van roof lose a lot in spring, autumn and winter compared with tilted panels. See the tilt angle calculator.
- Next, size your charge controller and battery bank.
Frequently asked questions
How many solar panels do I need to run a fridge in a van?
A 12 V compressor fridge typically uses 300–600 Wh a day. With 4 peak sun hours and 75% efficiency, that's 100–200 W of solar for the fridge alone.
How many solar panels to charge a 100Ah battery?
Replacing 100 Ah at 12 V (1,200 Wh) in one day with 4 sun hours needs about 400 W. Most people only replace part of the battery each day, so 200 W is common for a 100 Ah lithium battery.
Is it worth oversizing solar?
Often yes. Extra panel watts help on cloudy days and in winter, and an MPPT controller simply limits output on the brightest days.