RV Solar Calculator

A house array is designed once, for one location, and pointed as well as the roof allows. An RV array has none of those advantages, and the difference is larger than it first appears.

The panels are bolted flat, because a tilted array on a moving vehicle is a sail and a liability. Flat is nearly fine in midsummer when the sun is high, and poor in winter when it is low — a flat panel at latitude 40 in December sees the sun at a glancing angle and returns perhaps 60% of what a properly tilted one would.

The orientation is whatever the parking space dictates. A house faces south permanently; a van faces wherever there was room, and the good spots in a campground are shaded, because shade is what people want when they are not thinking about solar.

And the roof is small and full. After a rooftop air conditioner, two vents, a fan and an antenna, a long-wheelbase van might have room for three or four panels. That physical constraint is usually what actually determines array size, rather than any calculation of need.

Together these mean a van needs considerably more nameplate watts per usable amp-hour than a house does, and that beyond a certain point the roof simply runs out before the requirement is met.

The single most consequential decision in RV solar is which season to size for, and the honest default is winter.

The seasonal spread is severe. In the Pacific Northwest, winter peak sun hours can be a quarter of summer’s. Even in the Southwest — the best solar resource in the country — winter is around two-thirds of summer. Layer the flat-mounting penalty on top, which is itself worse in winter, and a flat array in a northern winter can deliver a fifth of its July output.

Meanwhile winter consumption goes up, not down. Nights are long so lights run longer, a diesel or propane heater draws fan power all night, people spend more time inside using devices, and the shorter days mean less driving and therefore less alternator charging.

An array sized on an annual average will therefore be comfortable exactly when it does not need to be and short exactly when it does. Sizing for winter is expensive, and the honest alternative is to size for summer and accept that winter means driving more, plugging in occasionally, or going south.

For a vehicle that moves, a DC-DC charger drawing from the alternator is frequently a better investment than the equivalent spend on panels, and it is consistently underrated.

A 40 amp DC-DC charger replaces a typical van’s entire daily consumption in two to three hours of driving. It works at night, in rain, under trees, and in December — none of which solar does. It costs a fraction of the equivalent solar capacity, takes up no roof space, and installs in an afternoon.

The device matters: a modern vehicle with a smart alternator varies its output voltage to manage the starter battery and will not reliably charge a house battery through a simple relay. A proper DC-DC charger takes whatever the alternator gives and produces a correct charge profile for the house bank, which for lithium is essential — connecting a lithium house battery directly to an alternator can draw more current than the alternator is designed to supply and cook it.

The trade-off is straightforward. A rig that drives most days barely needs solar. A rig that parks in one spot for a week needs every watt of it. Most people are somewhere in between, and the best system has both — solar to hold station, alternator to recover quickly when the battery is low.

The arithmetic says a certain number of panels. The roof says otherwise, and the roof wins.

Once a roof is reasonably covered, several things return more than another panel would.

Tilt brackets are the clearest. They recover most of the flat-mounting penalty and they do it in winter, which is when the shortfall exists. The catch is that they must be lowered before driving and, in practice, people stop bothering — which is an argument for a design that is genuinely quick to deploy rather than an argument against tilting.

An MPPT controller instead of PWM is the next, if the system still has one. With modern grid-voltage panels the difference is not marginal; PWM throws away more than half the array.

Moving loads to native DC removes the inverter from the equation. A 12 volt compressor fridge, DC lighting and USB-C outlets each avoid a conversion loss and, more importantly, remove the reason to leave a 20 watt inverter switched on around the clock.

And cleaning the panels is free. An RV array collects road film, tree sap and dust in a way a house roof does not, and because it is flat, rain does not wash it off — it pools and dries. Several percent sits under a hose.