A 200 W flat van-roof solar panel's output varies widely

Overview

A 200 W solar panel mounted flat on a van roof produces far less power in December than June, and the gap is much wider the further north you go [1]. Our own test, using the EU PVGIS model, measured what you can expect in nine real locations—from southern Spain to Norway [1].

Winter output is what sets your solar size

If you size your battery and solar for summer, you risk running short in winter. In Lisbon, a flat 200 W panel makes 319 Wh per day in December but 1,101 Wh in June [1]. In Edinburgh, December averages only 46 Wh a day, while June delivers 722 Wh [1].

Here are the measured daily outputs for a flat 200 W panel in December and June, using 15-year satellite data and the system losses you get on a real van roof [1]:

These numbers come from our PVGIS pull (2026-10-30) and include realistic system losses for a typical van install [1].

Tilting: The boost flat panels miss

Panels fixed flat on a van roof run hotter and catch the sun at a worse angle, especially in winter [1]. Our test found that tilting a panel to the ideal angle adds 18% to 27% more power over a year in most places—and much more in winter [1]. In December, the boost from tilting is dramatic:

• Tarifa: 77% more (372 up to 660 Wh/day)
• Munich: 133% more (140 up to 327 Wh/day)
• Amsterdam: 154% more (74 up to 189 Wh/day)
• Edinburgh: 255% more (46 up to 162 Wh/day) [1]

Mounting a panel flat is often the only practical option on a van, but these numbers show what is left on the table.

A US desert winter matches southern Europe

You might expect the best December output in southern Europe. But in our PVGIS test, Moab, Utah—a US high desert—matched or beat Lisbon and came close to Tarifa [1]. In December, a flat 200 W panel in Moab averaged 341 Wh per day, compared to Lisbon’s 319 and Tarifa’s 372 [1].

This means van lifers in the American Southwest can size for winter much as they would in southern Spain.

Why winter is the limiting factor

The winter-to-summer swing is most extreme in northern Europe. In Amsterdam, the panel delivers 74 Wh per day in December and 840 Wh in June: June is more than 11 times December [1]. In Edinburgh, the jump is even starker, with June delivering nearly 16 times more than December [1].

If you size your system for summer, you risk running dry when daylight hours shrink. For reliable off-grid power, you must base your battery and panel sizing on the lowest winter output for your location.

What the test data covers—and what it does not

Our PVGIS data gives 15-year averages for each place, from satellite sunlight records, factoring in a realistic system loss (14%), a flat panel installation, and local terrain [1]. It does not show the worst-case run of grey days, the effect of shade, snow, extra dust, or panels installed at a bad orientation [1]. It also does not include losses from a cheap charge controller or a van parked on a slope [1].

Questions readers ask

Why does tilting help so much in winter?

In winter, the sun is much lower in the sky. A flat panel catches far less sunlight, while a tilted one can face the sun directly and absorb more energy [1].

How much less should I expect on a run of cloudy days?

The test data gives long-term averages and does not show the worst week or month. Actual output on a string of grey days can be much lower than the December average [1].

Should I always size for December?

If you want reliable off-grid power year-round in Europe or the northern US, yes: sizing for December is safest [1]. If you only camp in summer, you can size for June instead.

Sources

1. The site owner's own test data (2026-10-30): Flat van-roof solar, December v June, nine places: our PVGIS pull, 2026-10-30