The roof is the specification
House solar starts from a load and works out how much array is needed. Vehicle solar cannot, because the roof does not grow. It starts from an area, subtracts everything already on it, and reports what is left. From there the only variables are consumption and the other charging sources.
What eats the roof is rarely the obvious thing. On an RV it is the air conditioner, the roof vent, the fan, the antenna, the skylight, and then the clearance each of those needs so it can be serviced and so it does not shade a panel edge. Thirty percent lost is a light estimate for a factory RV; fifty is common. On a van conversion the losses are smaller but the roof is narrower and the curvature means the outer four inches on each side is not really flat.
This is also why vehicle arrays are so often assembled from small modules. A single large module has better watts per square foot on paper and repeatedly fails to fit around a fan, while three 100 watt panels slot into the spaces that remain. Run the numbers for both panel sizes before buying, because the grid fit changes the answer more than the module efficiency does.
Flat panels on a hot roof
The derate for a vehicle array is considerably worse than for a house, and it is worth knowing where it goes.
| Loss | Typical | Why it is worse on a vehicle |
|---|---|---|
| Zero tilt | 8-20% | Panels lie flat because they have to. At mid latitudes that costs a tenth of the annual energy and much more in winter. |
| Module temperature | 8-15% | Mounted close to a metal roof with little airflow behind, in the sun, often over an uninsulated cavity. |
| Shading | Highly variable | The vent, the aerial, the awning arm, the tree you parked under because it was hot. |
| Soiling | 3-8% | Road film, dust, tree sap and bird mess, and a roof most owners clean rarely. |
| Controller and wiring | 3-8% | Short runs but low voltage, so drop matters, and often a PWM controller on a cheap install. |
Multiply those through and something between 55 and 70 percent end to end is what people actually measure. Anyone quoting 85 percent for a flat vehicle array has never metered one in August.
The alternator is usually the biggest single source
This surprises people who came for the solar. A 30 amp DC-DC charger at 12 volts puts 360 watt-hours into the bank for every hour the engine runs. An hour of driving is therefore worth roughly what a 200 watt panel produces in a good day, and in December it is worth considerably more than that. A travelling vehicle that moves most days has a fundamentally different energy problem from one parked for a fortnight, and systems designed for the first frequently fail at the second.
What the alternator will actually give is a separate question from what the battery would accept. Vehicle alternators are cooled by their own fan at engine speed and are designed for intermittent high output with a mostly-charged starter battery. Asking one to deliver sustained maximum current into a large lithium bank for several hours is a known way to cook it, and it is the reason DC-DC chargers with a current limit exist rather than direct paralleling. Lithium banks make this worse rather than better, because unlike lead-acid they do not taper their acceptance as they fill.
Reading the balance honestly
The number that decides whether a vehicle system works is the daily balance, not the battery size. If supply exceeds consumption, the bank recovers and its size only determines how many poor days you can absorb. If consumption exceeds supply, the bank is a countdown timer and doubling it doubles the countdown without changing the outcome.
Most people meet this problem in winter rather than at design time, because they sized on summer sun. An array producing 1,300 watt-hours in July at 4 peak sun hours produces around 650 in December at 2, while the fridge runs less but the heating fan, the lights and the water pump all run more. The gap between summer and winter on a vehicle is wider than on a house because there is no grid behind it and no room for a larger array.
When the balance is negative, three things fix it and only three: more array, more driving, or less consumption. Consumption is almost always the cheapest and it is almost always the last one tried. A compressor fridge on DC rather than an absorption unit, LED lighting, a diesel heater instead of a resistive one, and the inverter switched off when nothing needs it will often close a deficit that no amount of roof space could.
Marine is the same arithmetic with worse conditions
Boats face the same balance with three differences. Mounting area is scarcer and more of it is shaded by rigging, booms and biminis, and rigging shadows move constantly as the boat swings. Salt accelerates every kind of corrosion, so connections and mounts that would last a decade ashore do not. And the loads are different: refrigeration dominates, autopilot and instruments run continuously under way, and an electric windlass draws hundreds of amps for a few seconds in a way nothing on land does.
The daily balance still governs. What changes is that engine charging is more central, shore power is more intermittent, and the consequence of an empty bank is more serious than an early night.
Where to take the numbers next
Build the consumption figure properly rather than guessing it: the off-grid load audit calculator handles the AC and DC split and the inverter idle draw, which on a van is a large share of the day. Then check the discharge side with the battery runtime calculator and the recovery side with the battery charge time calculator. If the array is large enough to need series strings, the PV string voltage calculator matters even here, because a controller rated to 100 volts and four modules in series on a cold morning is the same failure a house system has. For the low-voltage cable runs, the voltage drop calculator in DC mode is the one to use, because at 12 volts a 3 percent budget is 0.36 volts and the conductors look absurd next to anything at 120 volts.
A PV array is energized whenever light falls on it. There is no switch on the panel that turns it off, and a module lying face up on the grass is a live source. DC arc faults do not self-extinguish the way an AC arc does, because DC current never passes through zero. A battery bank can deliver thousands of amps into a short circuit without any warning noise, and a dropped wrench across two terminals is a serious burn and fire hazard rather than a spark. Lithium cells that have been damaged, punctured, or charged below freezing can fail violently, and the failure is not always immediate. Nothing on this page is a wiring, commissioning or battery-assembly procedure, and none of it should be used as one.
Anything that ties a solar system to a building supply or to the grid is decided by your Authority Having Jurisdiction and your utility, not by a calculator. Interconnection, transfer switching, backfeed protection, disconnect placement, grounding, rapid shutdown and conductor sizing all fall to them, and in most places the work is licensed. Net metering terms, export credit rates and incentive programmes differ by state, by utility and by year, so treat nothing here as a statement of what yours will do.
Questions people ask
How much solar do I need for a van?
Work from consumption rather than from a target wattage. A conversion with a compressor fridge, LED lighting, a water pump, laptop and phone charging and a fan typically lands between 800 and 2,000 watt-hours a day, and the inverter idle draw is often 200 to 400 of that on its own. At 4 peak sun hours and 62 percent end-to-end efficiency, every 100 watts of flat roof array returns roughly 250 watt-hours a day, so a 1,400 watt-hour load needs about 560 watts of panel to cover it on solar alone in good conditions. Most vans cannot fit that and do not need to, because driving covers a large part of it.
Are flexible panels worth it?
They solve a real problem and create another. They weigh far less, they conform to a curved roof, and they can be bonded down without penetrating it, which matters on a fibreglass RV roof or a boat deck. Against that, bonding them flat to a surface removes the air gap that keeps a panel cool, so they run hotter and produce less, and their service life is generally shorter than a framed rigid panel, sometimes markedly so. On a van roof with room for rigid panels on a rail system, rigid usually wins on cost per watt-year. On a curved or weight-critical surface, flexible is the only option, and it should be priced as a consumable rather than a twenty-year fitting.
Do I need an MPPT controller or is PWM enough?
MPPT earns its cost when the array voltage sits well above the battery voltage, because it converts that excess voltage into extra charging current instead of throwing it away. With a nominal 12 volt panel on a 12 volt battery the difference is modest. With a 24 or 36 cell higher-voltage module, or two panels in series, MPPT can return 20 to 30 percent more energy, and on cold bright days more again. On a vehicle where roof area is the binding constraint, buying more energy from the same panels is usually the better trade, and MPPT also allows series wiring that keeps currents and conductor sizes down.
Will the alternator charge my lithium bank safely?
Not by direct connection, in general. A lithium bank does not taper its charge acceptance as it fills the way lead-acid does, so it will pull whatever the alternator can supply for as long as the engine runs, and vehicle alternators are not built for sustained full output. That is what a DC-DC charger is for: it limits current to something the alternator can deliver continuously and applies the correct charge profile for the chemistry. Some modern vehicles complicate this further with regulated or smart alternators whose output voltage varies with what the engine management wants, which a DC-DC charger also handles. The specifics belong to your vehicle and your battery documentation.
How many batteries do I need for a weekend without hookups?
Multiply your daily consumption by the number of days and divide by the usable depth of discharge, then check the balance separately. Two days at 1,400 watt-hours is 2,800 watt-hours, which at 80 percent usable on lithium is 3,500 watt-hours of nameplate, or roughly 290 amp-hours at 12 volts. On lead-acid at 50 percent it is 5,600 watt-hours, about 470 amp-hours, which is a lot of weight and floor space. But the more important check is whether the array and the drive home actually replace what you took out, because a bank that leaves full every Friday and returns at 30 percent every Sunday is fine, and one that never gets back above 60 percent is being slowly destroyed if it is lead-acid.