Tools · 02
Solar Sizing Calculator
Solar marketing quotes panel watts under laboratory sun. Your panels lie flat on a hot roof, sometimes under a cloud. This calculator uses conservative flat-mount peak-sun-hour values by region and season, applies a 30% real-world system loss, and tells you when the honest answer is "your roof isn't big enough — charge from the alternator."
Started from the battery calculator? Your daily watt-hours carry over automatically.
Inputs
Peak sun hours used: — · System losses applied: 30% (flat mount, heat, controller, wiring)
What this calculator assumes
- Flat mounting. The peak-sun-hour table is derated for panels lying flat on a van roof, not tilted to latitude. Tilting gains 10–25% (more in winter) if you're willing to climb up there.
- 30% system losses: cell temperature, charge-controller conversion, wiring, dust, and the morning you parked half under a tree. Sunny-brochure calculators use 15%; van reality doesn't.
- Peak sun hours are seasonal medians from NREL insolation data, rounded conservatively. A storm week is worse — that's what days of autonomy are for.
- Practical van-roof ceiling is ~400–600W once a fan, and usually a rack, take their share. Past that, the answer is a DC-DC alternator charger, not more glass.
Questions people actually search
How much solar do I need for my van?
Divide your daily watt-hour consumption by your location’s peak sun hours, then divide again by about 0.7 to account for real-world losses (flat mounting, heat, charge-controller and wiring losses, morning shade). A typical 200–300Ah build consuming 1,200Wh/day needs roughly 400–600W of panels in summer sun — and often can’t practically fit enough roof solar for winter in the northern US, which is why alternator charging matters.
Why do flat-mounted van panels produce less than rated?
Panel ratings assume perpendicular sun at 25°C cell temperature. A van roof is flat (worst-case angle outside the tropics), hot (output drops ~0.3–0.4% per °C above rating), and often partially shaded by a fan, rack, or trees. A 30% total system loss is a realistic planning number, and winter at high latitude is far worse.
Can I run a 12V air conditioner on solar alone?
Realistically, no — not from a van roof. Our field-implied draw for a budget 12V mini split on LOW is around 550W. Even 600W of roof solar (about the practical maximum on a high-roof van) only nets ~420W after losses in perfect sun, so the bank still drains while the sun shines and there is nothing left to recharge it. Solar is a range extender for A/C, not a power source. That math is exactly why our festival bank went flat with a charger assisting.
Is it better to add more solar or charge from the alternator?
Above roughly 400–600W, van roof space runs out and each additional solar watt gets expensive and awkward. A DC-DC alternator charger delivers 250–500W+ whenever you drive, in any weather, for less money. Most full-time builds want both: solar for stationary days, alternator for driving days and winter.