A van system that survives the days you stay put

Two of your three charging sources vanish the moment you stop moving. This works out the parked case separately, and prices every AC load at what the inverter really costs.

Home › Solar › RV Designer

Design an RV or van system

Where the energy comes from matters as much as how much — parked days and driving days are not the same.

Vehicle electrical work has its own hazards. This tool sizes energy and checks equipment limits. It does not specify wire gauges, fuse or breaker ratings, battery mounting, ventilation, or whether your alternator can sustain the current a battery-to-battery charger will ask of it — an alternator not rated for continuous high output can be destroyed by one. Lithium batteries must not be charged below freezing and need specific enclosure and ventilation arrangements. Have the installation done or checked by someone qualified for vehicle systems.

1 · How you travel

2 · What you run

Supply matters: a 12 V load draws straight from the battery, while an AC load goes through the inverter and costs more for the same job.

3 · Roof and panels

After vents, hatches and the aircon.

4 · Charging

Flat panels, no tilting, some shade — 70% is realistic.

5 · Battery and equipment

Where the energy goes

Where it comes from

Roof, battery and equipment

Your data stays in this browser. Everything on this page is calculated on your own device. Your electricity use, roof dimensions, prices and equipment details are never uploaded, stored on a server, or included in analytics.
Planning estimate — not an engineering or installation plan. Results depend on the information and assumptions you enter. Output, savings and equipment behaviour vary with weather, shading, tariffs, temperature, ageing and installation. Verify equipment limits against current manufacturer documentation. Permits, structural capacity, electrical protection, conductor sizing, grounding, rapid shutdown, fire access, utility interconnection and code compliance must be reviewed by qualified professionals and the applicable authorities.
How this is calculated

DC and AC loads cost different amounts. A 12 V load takes its watt-hours straight from the battery. The same job on an inverter costs watt-hours ÷ inverter efficiency, because the inverter loses some of what it passes. Adding them together as though they were equal understates the battery you need, and the error grows with how much of your load is AC.

solar = panel watts × peak sun hours × retained · alternator = amps × system volts × driving hours · shore = amps × volts × hookup hours ÷ 7

Parked days are calculated separately from the average. Alternator charge only exists while the engine runs, so a system that balances across a touring week can still go flat on the fourth day in one spot — which is usually when you are using the most.

Amp-hours are converted through energy: 100 Ah at 12 V, 50 Ah at 24 V and 25 Ah at 48 V are all the same 1.2 kWh. Series multiplies voltage, parallel multiplies amp-hours, and the energy is the same either way.

The parked day is the one that catches people

Most van systems are specified against an average. Solar brings in some, the alternator brings in some while you drive, an occasional hookup tops it up, and the arithmetic balances. Then you find a good spot and stay four days, and on the third evening the lights dim — because two of your three sources only exist when you are moving or plugged in.

So this works out the parked case on its own: solar alone against your daily need, the shortfall per day, and how many days the battery covers before it is empty. If you plan to stay put, that is the number your system has to survive, not the average.

Every AC load costs more than it says

A 60 W laptop on an inverter draws about 67 W from the battery at 90% efficiency, and the inverter itself has an idle draw whenever it is switched on. On a small system that adds up quickly. Anything that has a 12 V option — fridge, lights, fans, water pump, most charging — should use it, and the inverter should be switched off when nothing needs it. The table above shows what the inverter is costing you per day, which usually settles the argument.

Alternator charging deserves care

Pulling 40 or 60 amps continuously from an alternator that was designed to top up a starter battery is a good way to destroy it, and lithium batteries will happily accept everything the alternator can give until something fails. A battery-to-battery charger exists precisely to limit that, and whether yours is sized correctly for your specific alternator is a question for someone who knows the vehicle. This tool will happily calculate the energy; it has no idea what your engine can survive.