Van Solar Panels: How Many Watts You Need, Season by Season

What 200 W on the label really means

On the label: 200 W. On the roof of a van parked in Seattle in December: about 130 Wh a day on average, the equivalent of 40 minutes at full power. That’s what PVWatts, the solar calculator from the National Laboratory of the Rockies [7], works out for a 200 W panel lying flat. The same panel, in the same spot, produces about 930 Wh a day in June. Nearly seven times as much, without a single digit changing on the spec sheet.

The number on the label isn’t wrong. It’s a power rating measured under standard test conditions: 1,000 W of solar radiation per square meter of panel (about 93 W per square foot), at 77 °F (25 °C) [2]. It tells you how fast the panel produces in full sun, not how much it produces in a day. And it’s the second number that sizes a system: watt-hours per day, month by month, wherever you travel.

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

What a panel lying flat produces, month by month

We ran PVWatts V8 through its programming interface [7] for five cities, from Phoenix to Seattle. PVWatts comes from the National Laboratory of the Rockies (NLR), the U.S. Department of Energy lab known as NREL until December 2025 [8]. The settings: a 200 W panel lying flat, in other words tilted at 0°; open-rack mounting, with air behind it; and 14% losses, the default that PVGIS, the European Commission’s equivalent tool, suggests for cables, electronics, dirt, and module aging [2], a figure designed for grid-tied systems that we kept as is for a van. PVWatts uses typical-year weather data from the NLR’s National Solar Radiation Database [7]. Lying flat, the van’s orientation doesn’t matter: the panel faces the sky, not the south.

Two things stand out. In June, latitude matters less than you’d think: from Boston to Seattle, a 200 W panel delivers 920 to 990 Wh a day, and Phoenix tops out around 1,280 Wh. In December, the gap opens up, and the farther north you go, the wider it gets: between June and December, output is divided by 2.4 in Phoenix, 3.7 in Boston, and nearly 7 in Seattle.

Geometry explains most of it. At solar noon on December 21, the sun climbs only 33° above the horizon in Phoenix, 24° in Boston, and 19° in Seattle: its rays hit a panel lying flat at a grazing angle, and the days are short. The typical-year data behind PVWatts also reflect the clouds actually observed by satellite.

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

The sample budget in our house battery guide covers two people, a compressor cooler, 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 math for Seattle. In June, 1 W of panel lying flat produces 4.6 Wh a day there: 495 divided by 4.6 gives about 110 W. In September, 3 Wh per watt: 160 W. In December, 0.67 Wh per watt: you’d need 740 W, nearly four times the panel on the label.

From April to August, 200 W covers the example’s 495 Wh a day on average in all five cities. With 300 W, the season stretches from March through October everywhere. In Phoenix and Denver, the numbers stay friendly all year: in December, 200 W covers the average in Phoenix, and 300 W in Denver.

In winter up north, the math is unforgiving. With the heater on, the example’s consumption climbs to 891 Wh a day: in December, you’d need about 340 W of panels in Phoenix, 710 W in Boston, and 1,300 W in Seattle. With standard modules, those 1,300 W would take up about 62 sq ft (5.7 m²) [4]. Charging while driving or from shore power then does most of the work, and solar only slows the drain. Our guide to camper vans in winter breaks down what a warm night costs, and how a sleeping bag rated for the right temperature lets you turn the heater down.

The battery, a buffer between sunny days and gray ones

A monthly average hides whole weeks of overcast skies. That’s the battery’s job: storing the surplus from the good days to get you through the gray ones. For this, PVGIS, the European Commission’s solar tool, which also covers North America [1], offers an off-grid calculator: it replays every year of data hour by hour, with the battery and consumption you give it, and counts the days when the battery reaches its cutoff limitThe state of charge the battery isn’t run below, to protect it: 20% here, meaning 80% of the capacity gets used. [2].

We ran it for the same five cities 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]. For these cities, PVGIS 5.3 uses ERA5 reanalysis rather than satellite data [2]. The tool also errs on the side of caution: it assumes an overall performance ratio of 0.67, battery included [2], versus 14% losses in the first calculation.

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In June, 200 W is plenty: the battery hits its limit on only 0 to 3% of days, depending on the city. In spring and fall, every extra 100 W counts. In Seattle in March, the battery reaches its limit on 62% of days with 200 W, 16% with 300 W, and 4% with 400 W; in October, 75%, 33%, and 11%. In December, nothing helps in Seattle: 100% of days, even with 400 W. In Phoenix, 300 W keeps the battery off its limit almost every day, December included.

The flip side shows up in summer: more panels also means more energy wasted once the battery is full. In Phoenix, with 400 W, PVGIS counts about 1,800 Wh a day in June that neither the battery nor your appliances can absorb. Extra watts only help in the months when output falls short.

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

Panels heat up, and a hot panel produces less. According to PVGIS, modules with no air behind them can run up to 27 °F (15 °C) hotter in full sun [2]. The tool knows nothing about vans: the closest case to a flexible panel glued straight onto the roof is its “building-integrated” mounting. With that setting, output drops about 5% in July in Phoenix and about 4% in Seattle. 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 to 2.8 times as much in December, according to PVGIS, but 23 to 41% less in June: lying flat, a panel is set up for summer. And even tilted, 200 W would give only about 310 Wh a day in Seattle in December, well short of the example’s 495 Wh.

That leaves the losses the tools can’t see. PVGIS accounts for terrain blocking the horizon [3], but not for the tree, the building, or the neighboring RV shading your campsite. It lumps dirt, and sometimes snow, into its general losses [2]. Parked in the shade of a stand of trees, a panel will produce less than these tables, which assume a sky with no nearby obstacles.

Before installing: 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 meter of the test conditions, 200 W takes up about 9 to 11 sq ft (0.8 to 1.1 m²), and 400 W about 17 to 23 sq ft (1.6 to 2.1 m²). Those figures apply to large modules: check the efficiency on your own panel’s spec sheet, then measure the open space between the roof vent, the fan, and the roof rack.

Weight next: panels, brackets, and cables add to the vehicle’s weight and eat into its payload, the margin between its curb weight and its gross vehicle weight rating (GVWR). Keep an eye on it if your van is already loaded close to that limit.

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A solar panel is sized for the darkest month of your travels: divide what you use each day by what 1 W of panel produces that month, wherever you’re headed.

Worth watching

A 200-watt rooftop setup shown in use while boondocking, with the factors that decide whether it’s enough and a way to check it against your own power use: the real-world side of this guide’s question.

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Julien de Jurquet

Vans, RVs, biking and calculators

Julien de Jurquet writes for Outfarer about camper vans, RVs, biking, and calculators. He reads manuals cover to cover and never lets a number slide without its unit, so your choices rest on math, not promises.

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