The math
Solar watts needed ≈ daily Wh ÷ (sun hours × system factor)
- Daily Wh comes from your load list. If you don't have one yet, start with the power question first.
- Sun hours means peak-sun-equivalent hours, not daylight hours. A long summer day might give you five; a short winter day, two or less.
- System factor covers heat, dust, panel angle, wiring and controller losses. Flat roof panels in real life rarely hit their label. A planning factor of 0.75 is a reasonable starting assumption.
What it depends on
- What you use. Solar follows the load list. Cut 300 Wh a day and you need less panel.
- Where and when you travel. Summer in the desert and November in the mountains are different worlds. Look up solar resource for the places and months you'll be.
- How much you drive. If you drive an hour most days, the alternator through a DC-DC charger can cover a big share, and solar becomes the parked-day backup.
- Roof space. The fan, any vents, the rack and its feet, and panel sizes decide what fits. A panel that shades your fan opening or another panel costs more than it adds.
- Shade. Parking under trees to stay cool works against solar. Pick one.
- Flat vs tilted. Most vans run flat. Tilting helps in winter but adds hardware and hassle.
Worked sample (planning estimates)
Same sample build as the power page: about 1,200 Wh a day.
| Scenario | Sun hours (assumed) | Math | Panel needed |
|---|---|---|---|
| Summer, open sky | 5 | 1,200 ÷ (5 × 0.75) | about 320 W |
| Winter | 2.5 | 1,200 ÷ (2.5 × 0.75) | about 640 W |
The sun hours here are sample assumptions, not data for any location. Swap in your own.
In practice, a 400 W array covers summer in this sample with a little room. Winter is where it falls short, and that's normal. The fix is usually not more panels than the roof can hold. It's charging from the alternator while you drive.
Add the alternator. A 30 A DC-DC charger putting out about 14.2 V delivers around 420 W. One hour of driving is roughly 420 Wh, about a third of this sample day. Two hours of driving and winter solar together close most of the gap.
That's the pattern: solar for parked days in good sun, alternator for winter and travel days, shore power when it's around.
Check before you buy
- Panel voltage and current against the charge controller's input limits in its manual, including cold-weather voltage.
- Roof layout drawn to scale with the fan, rack feet and panel sizes.
- The cable path from roof to controller, decided before insulation and ceiling go in.
When to bring in a professional
Roof mounting means penetrations or a rack, and the solar run ties into the rest of the electrical system. If the rack or panel mounts touch roof structure, or you're not confident in the wiring from controller to battery, have an upfitter or qualified electrical tech look at it first.