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About VanSolarCalc

VanSolarCalc is a free tool that sizes an off-grid electrical system for a camper van or RV. Enter your appliances and where you travel, and it works out the solar array, battery bank, charge controller, inverter, wire gauge, and fuses you need, then estimates the cost and builds a printable parts list.

Why it exists

Most van-build advice is a mix of forum rules of thumb and product marketing. The result is systems that are either wildly oversized and expensive, or dangerously undersized. VanSolarCalc replaces guesswork with the same arithmetic a good installer would use, shown transparently so you can check every step. There is no login, no upsell, and no affiliate steering toward one brand. Just the numbers.

How the math works

Every result comes from a documented formula, not a lookup of "what other people bought":

  • Solar array: daily watt-hours ÷ (peak sun hours × 0.75), where 0.75 is a real-world derate for wiring loss, controller efficiency, heat, and imperfect angle.
  • Battery bank: daily Wh × days of autonomy ÷ depth of discharge (0.8 for LiFePO4, 0.5 for AGM), with a cold-weather derate option.
  • Charge controller: array watts ÷ system volts × 1.25, rounded up to a standard MPPT amperage.
  • Wire and fuse: ABYC E-11 105 °C ampacity to cover the fuse, and a 3% voltage-drop check over the round-trip cable length, and the thicker of the two results wins.

Standards it follows

The battery-to-inverter cable is sized to the ABYC E-11 marine standard, which is the correct reference for 12/24V DC vehicle wiring, not the household NEC table, which gives different numbers. Voltage drop is always calculated on the round-trip length, because current flows down the positive conductor and back through the negative.

Who maintains this

VanSolarCalc is written and maintained by Frank Zale, who built his own camper van from scratch and is responsible for the sizing methodology and for every number the tool produces.

That build is why the tool is shaped the way it is. Sizing a van system on paper and actually installing one are different problems: the calculation tells you a cable needs to be 4 AWG, but it does not tell you that the run is longer than you measured, that the fuse has to sit within a few inches of the battery terminal, or that a fan and two roof vents will take the space you had earmarked for a third panel. The pages here try to answer both: the number, and the thing that goes wrong when you fit it.

A word on why the method is written out above rather than summarized. Most van-electrical advice asks you to trust the person giving it. This site is built so you do not have to: each result traces to a named standard or a stated formula, the intermediate arithmetic is shown, and the assumptions behind it (0.75 solar derate, 0.8 LiFePO4 depth of discharge, 3% voltage drop on the round trip) are printed rather than buried. If a number here looks wrong, you have everything you need to check it, and that is deliberate.

Corrections are genuinely welcome. If you find an error in a formula, a constant, or a wire-table figure, email hello@vansolarcalc.com and it will be fixed and noted. Reader-reported corrections have a standing place in the changelog.

Honest limitations

The cost estimate is a planning range, not a quote. Component prices move, and your exact parts and shipping will differ. The tool sizes a system; it is not a substitute for a qualified installer signing off on a high-current DC build. Always fuse every positive run at its source and torque lugs to spec.

Questions or corrections? Get in touch. The calculator is on the home page whenever you are ready to size a build.