Campervan solar panels: how many watts to fit for each season

What 200 W on the label actually means

On the label: 200 W. On the roof of a campervan parked in Lille in December: about 100 Wh a day on average, the equivalent of half an hour at full power. That is what PVGIS, the solar tool of the European Commission’s Joint Research Centre [1], calculates for a 200 W panel lying flat. The same panel, in the same place, produces nearly 880 Wh a day in June. Nine times as much, without a single comma changing on the data sheet.

The figure on the label is not wrong. It is a power output measured under standard test conditions: 1,000 W of solar radiation per square metre of panel, at 25 °C [2]. It tells you how fast the panel produces in full sun, not how much it produces in a day. Yet it is the second figure that sizes a system: watt-hours per day, month by month, wherever you travel.

So this guide follows three steps: what each watt of panel produces depending on the month and the latitude; the wattage that covers your consumption; then the battery, which bridges the gap between sunny days and grey ones. Your consumption itself is worked out appliance by appliance, as our leisure battery guide explains.

What a panel lying flat produces, month by month

We queried PVGIS 5.3, the version currently in service [1], through its programming interface [3], for six cities from Marseille to Edinburgh. The settings: a 200 W crystalline silicon panel lying flat, in other words tilted at 0°; mounted on brackets with air behind it; and the 14% losses that PVGIS suggests by default for cables, electronics, dirt and module ageing [2], a figure designed for grid-connected systems that we have kept as it is for a campervan. The averages cover 2005 to 2023, based on satellite data from the SARAH-3 database [2]. Lying flat, the van’s orientation no longer matters: the panel faces the sky, not the south.

Two things stand out. In June, latitude makes little difference: from Munich to Hamburg, a 200 W panel delivers 840 to 880 Wh a day, and Edinburgh is not far behind. In December, everything collapses, and all the more so the further north you go: between June and December, output is divided by 4.5 in Marseille, by 9 in Lille and London, and by 17 in Edinburgh.

Geometry explains most of it. At solar noon on 21 December, the sun climbs only 23° above the horizon in Marseille, 16° in Lille and 11° in Edinburgh: its rays reach a panel lying flat at a grazing angle, and the days are short. The PVGIS averages also take account of the clouds actually observed by satellite.

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200, 300 or 400 W: the wattage that covers your consumption

The example budget in our leisure battery guide covers two people, a compressor cool box, lighting, a laptop and two phones: about 495 Wh a day in summer, and 891 Wh in winter with a diesel air heater. That leaves finding the panel wattage that produces as much, month after month.

Let’s do the maths for Lille. In June, 1 W of panel lying flat produces 4.4 Wh a day there: 495 divided by 4.4 gives about 110 W. In September, 2.7 Wh per watt: 180 W. In December, 0.48 Wh per watt: you would need 1,000 W, five times the panel on the label.

The table answers the question in the headline, for this example. From April to August, 200 W covers the 495 Wh a day on average in all six cities. To stretch the season to March and September, allow 300 W, as far north as Edinburgh. With 400 W, October works too, except in Edinburgh.

In winter, the sums are unforgiving. With the heater on, the example’s consumption rises to 891 Wh a day: in December, you would need nearly 700 W of panels in Marseille, 1,800 W in Lille and 4,000 W in Edinburgh. With standard modules, those 4,000 W would cover nearly 18 m² [4]. In winter, charging while driving or from a hook-up does most of the work, and solar only slows the drain. Our guide to campervans in winter sets out what a warm night costs, and how a sleeping bag rated for the right temperature lets you turn the heating down.

The battery, a buffer between sunny days and grey ones

A monthly average hides whole weeks of low cloud. That is the battery’s job: storing the surplus from the good days to get through the grey ones. For this, PVGIS offers a second tool, designed for off-grid systems: it replays every year of data hour by hour, with the battery and consumption you give it, and counts the days on which the battery reaches its cut-off limitThe state of charge below which the battery is not run down, to spare it: 20% here, meaning 80% of the capacity is used. [2].

We ran it with the example’s battery and consumption: 100 Ah of lithium at 12 V, or 1,200 Wh, cut off at 20% as PVGIS suggests for lithium, and 495 Wh used every day, mostly in the evening [2]. The tool errs on the side of caution: it assumes an overall performance ratio of 0.67 for the whole system, battery included [2], whereas the first calculation only took off 14% for losses.

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In June, 200 W is enough: the battery hits its limit on only 0 to 5% of days, depending on the city. In spring and autumn, every extra 100 W counts. In Lille in March, the battery reaches its limit on 76% of days with 200 W, 29% with 300 W and 11% with 400 W; in October, 94%, 46% and 13%. In December, nothing helps in the north: 100% of days in Lille as in Edinburgh, even with 400 W. In Marseille, 400 W brings December down to one day in four.

The flip side shows in summer: more panels also means more energy wasted once the battery is full. In Marseille, with 400 W, PVGIS counts about 1,550 Wh a day in June that neither the battery nor the consumption can absorb. Extra watts therefore only help in the months when output falls short.

Heat, tilt and shade: what changes output on a campervan

Panels heat up, and a hot panel produces less. According to PVGIS, modules with no air behind them can run up to 15 °C hotter in full sun [2]. The tool knows nothing about campervans: the closest case to a flexible panel glued straight onto the roof is its ‘building-integrated’ mounting. With that setting, output drops by 3 to 4% in July, depending on the city. Real, but modest next to the gap between June and December.

Tilting the panel changes more. Raised to 60° facing south, the same panel would produce 2.6 to 4 times as much in December, according to PVGIS, but 15 to 26% less in June: lying flat, a panel is set up for summer. And even raised, 200 W would give only about 270 Wh a day in Lille in December, well short of the example’s 495 Wh.

That leaves the losses the tool cannot see. PVGIS allows for the terrain that blocks the horizon [3], but not for the tree, the building or the neighbouring motorhome casting shade over the car park. It lumps dirt, and sometimes snow, into its general losses [2]. Parked in the shade of a hedge, a panel will therefore produce less than these tables, which assume a sky clear of any nearby obstacle.

Before fitting: space, weight and the installer

Space first. Crystalline silicon modules from the ten largest manufacturers averaged 22.7% efficiency at the end of 2024, ranging from 18.9 to 24.8% depending on the model [4]. Under the 1,000 W per square metre of the test conditions, 200 W therefore takes up 0.8 to 1.1 m², and 400 W from 1.6 to 2.1 m². These figures apply to large modules: check the efficiency on your own panel’s data sheet, then measure the free space between roof light, vent and roof bars.

Weight next: panels, brackets and cables add to the vehicle’s mass and eat into its payload, something to watch on a motorhome already close to its maximum weight. Our article on 4.25-tonne motorhomes and the car licence explains these weights.

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A solar panel is chosen for the darkest month of your travels: divide what you use each day by what 1 W of panel produces that month, wherever you are going.

Julien de Jurquet

Campervans, motorhomes, cycling and calculators

Julien de Jurquet writes for Outfarer about campervans, motorhomes, cycling and calculators. He reads instruction manuals to the very end and never lets a figure through without its unit, so that your choices rest on sums rather than promises.

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