RV, Van & Boat Solar Calculator

How to size a solar system for an RV, van or boat

Mobile solar — on an RV, camper van, or boat — follows the same principles as a home off-grid system, just scaled down and with tighter constraints on space and weight. The goal is to keep your house battery charged from the roof so you can camp, travel or anchor without hooking up to shore power or running a noisy generator. As always, it starts with an honest tally of what you use each day, then works back to the panels and battery that will sustain it.

Solar has transformed life on the road and water, turning vans, motorhomes and boats into places you can live and work without being tethered to a hookup post or listening to a generator drone. But a mobile rig is the most space- and weight-constrained solar system you’ll ever design, which makes careful sizing more important than at home, not less — there’s no room to simply add another panel later, and every kilogram of battery competes with everything else you carry. The discipline that pays off is the same one that underpins any off-grid system: know exactly what you draw in a day, then build the smallest array and battery that reliably cover it with a little margin. The calculator below works through that for your rig, and the guidance that follows explains the choices — lithium versus lead-acid, panel placement, and backup charging — that matter most when space is tight.

Start with your daily energy use

List your 12V and AC loads: lights, water pump, fridge (the biggest single draw in most rigs), fans, phone and laptop charging, and any inverter-run appliances. Multiply each by its daily hours for watt-hours, and sum. A modest van might use 1,000–2,000 Wh a day; a larger RV with a powerful fridge and electronics, considerably more. The fridge usually dominates, so its efficiency and duty cycle matter most — and a compressor fridge cycles on and off rather than drawing continuously, so don't assume 24-hour draw.

Panels and the space constraint

Roof space, not budget, is usually the limit on a mobile rig. Size the array to replace your daily energy use within your typical sun-hours, but expect to fit less than ideal — most vans manage 200–600 W of panels, larger RVs more. Because mobile systems often travel to varied climates and park in partial shade, real output is lower than a fixed home array, so size generously and don't bank on perfect conditions. Tilting portable panels toward the sun when parked can meaningfully boost a space-limited setup.

Battery and the case for lithium and higher voltage

Mobile systems benefit especially from LiFePO4 batteries: their high usable depth of discharge (80–95% vs ~50% for lead-acid), lighter weight and longer life are worth a lot when space and payload are scarce. Size the battery to your daily use times the days of cloud you want to ride out — often just 1–2 days, since you can usually drive (and alternator-charge) or move to sun. Many modern rigs also use the vehicle alternator and shore power as additional charging sources, so solar need not carry the whole load alone. Keep wiring runs short and consider 24V for larger systems to reduce current and cable size.

Charging from more than the sun

The most reliable mobile setups treat solar as one of several charging sources rather than the only one. While parked in sun, the panels do the work; while driving, a DC-to-DC charger from the vehicle alternator tops up the house battery; and at a campsite with hookups, shore power fills any gap. This layered approach means your solar can be sized for typical conditions rather than the worst case, saving precious roof space and payload. Think of solar as your default free source and the alternator and shore power as backups for cloudy stretches or heavy use.

Frequently asked questions

It depends on your daily energy use and roof space. A simple van running lights, a fridge and charging might need 200–400 W; a larger RV with more appliances, 600 W or more. Enter your loads above for an estimate, and fit as much as your roof allows.

Size it to your daily watt-hours times the days of autonomy you want (often 1–2, since you can drive to charge or follow the sun), divided by usable depth of discharge. LiFePO4 is strongly preferred for its light weight, high usable capacity and long life in a mobile setting.

It's demanding — RV air-conditioners are heavy loads with large surges, needing a substantial battery and inverter and more panels than most roofs hold. Many people run AC from shore power or a generator and use solar for everything else, or rely on it only when the sun is strong.

Often as backup. Solar handles day-to-day needs in good conditions, but for extended cloudy stretches, heavy loads or air-conditioning, shore power, the vehicle alternator, or a small generator fills the gap — a hybrid approach that mirrors home off-grid design.

Many people do — a basic system (panels, charge controller, battery, inverter) is within reach of a careful DIYer, and our wiring and charge-controller calculators help size the parts. Take care with roof mounting, fusing and battery safety, and size the inverter for any surge loads. When in doubt, especially with lithium batteries and high currents, consult a professional.