The most common solar question for US van and RV builds: how many watts is actually enough? It depends on your usage and where you'll be parked — here's how to work it out.
Size your solar in 5 minutes
We'll calculate the panel wattage that matches your daily usage and battery — free, with a wiring diagram.
The formula
Solar watts ≈ daily Wh ÷ (peak sun hours × 0.75 system efficiency)
The 0.75 factor accounts for real-world losses — panel angle, temperature, dust, partial shading, and wiring/controller efficiency. Rated panel wattage is measured under lab conditions you'll rarely see in the field.
Peak sun hours vary a lot across the US
| Region | Summer peak sun hours | Winter peak sun hours |
|---|---|---|
| Desert Southwest (AZ, NM, southern CA) | 5.5-6.5+ | 4-5 |
| Southeast / Gulf Coast | 5-5.5 | 3.5-4.5 |
| Midwest / Mid-Atlantic | 4.5-5 | 2.5-3.5 |
| Pacific Northwest / Northeast | 4.5-5 | 2-3 |
If you travel seasonally (e.g., south for winter), size around the conditions you'll actually camp in most. If you stay in one region year-round, size for your worst realistic month — or accept that solar alone won't fully cover winter and lean on the charging systems guide for DC-DC and shore power.
Worked examples
Weekend van, ~700Wh/day, traveling in sunny regions (5 peak sun hours):
700 ÷ (5 × 0.75) ≈ 187W → a single 200W panel.
Full-timer, ~1,500Wh/day, mixed regions (4.5 peak sun hours):
1,500 ÷ (4.5 × 0.75) ≈ 444W → a 400-500W array (two 200-250W panels).
Heavy use with inverter, ~2,200Wh/day, average 4 peak sun hours:
2,200 ÷ (4 × 0.75) ≈ 733W → 600-800W array, likely paired with DC-DC for travel days.
More battery often beats more solar past a point
Roof space and budget are finite. Once you're past ~400-600W, many builders get more value from added battery capacity and a DC-DC charger than from squeezing more panels onto the roof — see the solar setup guide.
Don't forget the MPPT controller
Your charge controller needs to be sized for the array, and the array's cold-weather voltage needs to stay within its input limit — covered in the solar setup guide and MPPT vs. PWM controllers.
Related guides
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