Rig Amps may be paid through affiliate links to brand stores. As of November 7, 2026, no affiliate program has accepted the site yet. Panels are sorted by daily energy, never by commission. Affiliate disclosure.
Measure the largest free rectangle on your roof, between vents, the fan and any rack, in feet. The tool tries each of the 15 panels in our list, in both orientations, with the dimensions their makers publish, then multiplies the watts by the peak sun hours of your state from the National Laboratory of the Rockies (formerly NREL).
How the tool counts
- Fit: panels per row = whole number of (roof length + gap) / (panel length + gap), times rows across the width the same way; the tool keeps the better of the two orientations. Dimensions are the published ones, such as BougeRV's "51.57×30.20×1.38 in" for its 200W 16BB panel or Renogy's "37.09 x 24.13 x 0.10in" for the Flex ShadowFlux 100W.
- Daily energy: total watts x peak sun hours x (1 - losses).
- Sun: NLR Solar Resource Data API v1, average Global Horizontal Irradiance (avg_ghi), "kWh/m2/day", "averaged from hourly model output over 12 years (1998-2009)", for 0.1-degree cells (about 10 km). We read it at the capital of each state on November 7, 2026; the API's free demo key allows few calls per hour, so the list grows over time. For another place, type its value.
- Controller: the cheapest MPPT controller whose page publishes a 12 V solar limit at least equal to the array watts: Renogy Rover Li 40A "12V/520W", Rover 60A "800W/12V", BougeRV Sunflow 30A "450W/12V", 40A "600W/12V", 60A "900W/12V".
Worked example [calc], checked by hand: a 10 x 5 ft area is 120 x 60 in. Renogy's 200W bifacial is 51.7 x 30.1 in: lengthwise 2 x 1 = 2 panels (120 / 51.7 = 2.3, 60 / 30.1 = 1.99), crosswise 3 x 1 = 3 panels (120 / 30.1 = 3.98, 60 / 51.7 = 1.16). Three panels, 600 W. In Phoenix over the year (5.78 h): 600 x 5.78 = 3,468 Wh a day before losses. That is more than the 520 W the Rover Li 40A accepts at 12 V, so the tool points to the next controller up.
Sun by state, flat roof
| State (capital) | Year average | December | June | 200 W panel, December [calc] | 200 W panel, June [calc] |
|---|---|---|---|---|---|
| Arizona (Phoenix) | 5.78 h | 3.18 h | 8.31 h | 636 Wh | 1,662 Wh |
| California (Sacramento) | 5.07 h | 1.84 h | 8.12 h | 368 Wh | 1,624 Wh |
| Colorado (Denver) | 4.83 h | 2.26 h | 7.21 h | 452 Wh | 1,442 Wh |
| Florida (Tallahassee) | 4.79 h | 2.8 h | 6.07 h | 560 Wh | 1,214 Wh |
| Nevada (Carson City) | 5.27 h | 2.11 h | 8.17 h | 422 Wh | 1,634 Wh |
| Oregon (Salem) | 3.77 h | 1.06 h | 6.13 h | 212 Wh | 1,226 Wh |
| Texas (Austin) | 4.74 h | 2.71 h | 6.63 h | 542 Wh | 1,326 Wh |
| Utah (Salt Lake City) | 4.71 h | 1.74 h | 7.49 h | 348 Wh | 1,498 Wh |
| Washington (Olympia) | 3.37 h | 0.88 h | 5.64 h | 176 Wh | 1,128 Wh |
How we compare: criteria, sources and dates
Source: NLR (National Laboratory of the Rockies, formerly NREL) Solar Resource Data API v1, avg_ghi, state capital coordinates rounded to 0.01 degree, read November 7, 2026. [calc] columns: 200 W x peak sun hours, before losses.
Roof work. Drilling, sealing and cable entries on a vehicle roof affect water tightness and the structure. Solar strings can carry more than 12 V: an array of panels in series can exceed 50 V on the spec sheets we read (BougeRV 300W: "Open Circuit Voltage Voc (V) | 44.2±5% V" per panel). Have it installed by a qualified installer and follow the panel and controller manuals.
Next: how much solar a camper van needs, and the full battery comparison.
Frequently asked questions
What are peak sun hours?
The daily solar energy on a surface, in kWh per square meter, is the same number as the hours of full 1,000 W per square meter sun. NLR's Solar Resource API returns it for a flat surface as avg_ghi, for each month and the year. A flat van roof is close to that case.
Why is my winter number so low?
Because the sun is low and the days are short. On a flat surface NLR gives 1.84 hours a day in Sacramento in December against 8.12 in June. Tilting panels toward the sun helps in winter; a flat-mounted van panel cannot.
Should I deduct losses?
Yes, in real life. Heat, wiring, the controller and dirt all take a share. No maker in our list publishes a single loss figure for a whole van system, so the tool starts at 0% and lets you set yours.
Rigid or flexible panels?
On the pages we read, rigid panels pack more watts per square foot (200 W in about 10.4 sq ft for Renogy's ShadowFlux) and flexible ones weigh far less (BougeRV lists 7.9 lbs for its Arch Pro 200W). The table below shows watts per square foot for every panel.