RV & Van Solar Power Calculator

Add up the watt-hours each device uses in a day (watts × hours), then divide by your peak sun hours and a 0.75 system derate that accounts for wiring, controller and panel-temperature losses. A van pulling 1,200 Wh/day in a 4.5-sun-hour location needs about 356 W of panel, which in practice rounds up to a 400 W panel or two 200 W panels. Roof space, shading and how often you drive or run a generator all shift the real number, so treat this as a planning estimate.

Multiply your daily watt-hours by the number of cloudy or no-drive days you want to ride out (autonomy), then divide by the system voltage and the usable depth of discharge — about 0.8 for lithium (LiFePO4) and 0.5 for lead-acid. For 1,200 Wh/day, 2 days of autonomy and a 12 V lithium bank that is (1,200 × 2) / (12 × 0.8) = 250 Ah. Lead-acid of the same usable size would need roughly 400 Ah because you only safely use half of it.

12 V is the default for most vans and RVs because the appliances, fans and water pumps are 12 V native. 24 V becomes attractive on larger rigs (above roughly 1,500–2,000 W of array or long cable runs) because doubling the voltage halves the current, which lets you use smaller, cheaper wire and reduces voltage drop. If you go 24 V you will need a DC-DC converter to feed 12 V loads. This calculator handles either; the battery Ah and controller amps fall as voltage rises.

Controller current is the array wattage divided by the battery voltage, times a 1.25 safety margin for cold-weather over-current: 356 W on a 12 V bank is about 37 A, so a 40 A MPPT controller. MPPT controllers harvest 10–30% more than PWM by converting excess panel voltage into charge current, which matters most when the panel voltage is well above the battery voltage. The tool returns both the raw amps and the next standard controller size.

Peak sun hours swing widely between summer and winter. If you live in the rig year-round, sizing on the annual average leaves you short in December when the sun is low and the days are short; use the worst-month figure for the regions you park in. If you only travel in summer or chase the sun, the average — or even a summer figure — is fine. The reference table linked below lists representative peak sun hours by US state.

Go device by device: multiply each load’s watts by the hours per day you run it, then add them up. A 50 W fridge running effectively 12 hours a day is 600 Wh; a 10 W fan for 8 hours is 80 Wh; lights, a water pump and charging phones add the rest. Sum every item to get the daily watt-hours this calculator asks for. When you are unsure, round up — undersizing the array and battery is the more common and more annoying mistake in a van build.

Two ways. The 0.75 system derate baked into the array formula covers everyday losses — wiring resistance, controller inefficiency, dust and the way panels make less power when hot. Cloudy days are handled separately by the days-of-autonomy input, which sizes the battery to carry you through low-sun stretches without charging. For serious off-grid use, combine generous autonomy with a worst-month sun figure, and keep an alternator or shore-power charger as a backup for long overcast spells.