It follows from the daily consumption in watt-hours, not from the roof area. Add up what runs each day, decide how much of it solar should cover in the worst month you will travel, and the panel figure comes out of that arithmetic.
Panel wattage is the last number in this calculation, not the first, and starting at the wrong end is why so many vans carry solar that never quite keeps up.
Start with the daily load
Every device, its draw in watts, and the hours it runs. The fridge dominates and is the hardest to estimate, because a compressor cycles rather than running continuously — an energy meter on the fridge for two days answers it better than any label.
Turn it into a daily yield requirement
A panel does not produce its rating for the hours the sun is up. The useful planning figure is daily energy yield, which varies with latitude, season, tilt and shading, and which published irradiance data describes far better than a rule of thumb.
Then look at the roof
Space, not ambition, sets the ceiling. Vents, hatches, aerials and a habitation door all take room, and a panel that overhangs a roof edge is a wind problem. This is where watts per square metre and mounting hardware start deciding the design.
Then decide what fills the gap
Almost every van ends up with more than one charging source: solar for the days at anchor, a DC-DC charger for the days spent driving, and a mains charger for the nights on a hookup. Sizing solar to cover the darkest week alone usually produces a roof nobody can fit.
What people ask before they order
Why does the worst month matter so much?
Because a system sized for summer runs out in November. Daily yield in midwinter can be a small fraction of the midsummer figure at the same latitude, and that ratio is what decides whether a second charging source is needed.
Is roof space or panel rating the real limit?
On most vans, space. Which is why the comparison worth making between two panels is watts per square metre rather than watts alone.
Last reviewed 16 September 2026