How much sun will your van’s solar really get?

What are peak sun hours?

Sunlight strength changes through the day. Peak sun hours add up a whole day’s sunshine and express it as hours of full midday sun (1,000 watts per square metre, the strength solar panels are rated at). Five peak sun hours means the panel got the same energy as five hours at full strength, even if the sun was up for twelve.

The numbers here are NASA’s global horizontal irradiance: the sunlight landing on a flat, level surface, in kilowatt-hours per square metre per day. On a flat panel, 1 kWh/m² a day is exactly one peak sun hour. They include cloudy days, because they are long-term averages of real weather.

Flat panels versus tilted panels

Most van panels lie flat on the roof. In summer, when the sun is high, flat works well. In winter the sun stays low in the sky, so a flat panel catches it at a slant and gets much less. Tilting a panel to face the sun can gain a lot in winter, especially far from the equator, but it means parking to face the sun and remembering to lower the panel before you drive. Because these numbers are for flat panels, they are the realistic figure for most vans.

From sun hours to watt-hours

• Daily Wh into the battery = panel watts × peak sun hours × 0.74.
• The 0.74 covers real-world losses: dirt, shade, wiring and other system losses (× 0.86), heat (flat roof panels run hot) (× 0.90), charge controller and battery charging (× 0.95). The first is NREL PVWatts’ default 14% system loss; the heat figure uses PVWatts’ −0.47% per °C for standard panels; the charging figure is our allowance.
• Example: 400 W of panels in Phoenix in June (8.5 hours) gives about 400 × 8.5 × 0.74 ≈ 2,500 Wh a day. In December (3.1 hours) it is about 910 Wh.

Good to know

• Each value is an average over a grid square roughly 100 km across. Mountains, fog belts and coastlines can differ from the town itself.
• Shade from trees, buildings or a roof rack cuts output much more than you might expect.
• Far north in winter (Alaska, northern Norway) there is almost no sun. Plan to charge from driving or hookups.
• Averages hide bad weeks. Size your battery for a few cloudy days in a row with the solar and battery calculator.

Sources

• NASA POWER Climatology API (v2.10.1), NASA Langley Research Center: ALLSKY_SFC_SW_DWN, all-sky surface shortwave downward irradiance (kWh/m²/day), CERES SYN1deg, 25-year average, January 2001 to December 2025. Retrieved 2026-11-01 through the climatology API for each place’s coordinates. Values are rounded to two decimals and shown to one.
• NREL, PVWatts Version 5 Manual (2014): default system losses and module temperature coefficient.