Methodology & sources
By Frank Zale · Last updated
Every number VanSolarCalc produces comes from a stated formula and a named standard. This page lists all of them, so you can check the tool rather than trust it. If you find an error, tell us and it gets fixed.
The five calculations
1. Daily energy
Wh/day = Σ (device watts × hours per day)
Everything else follows from this figure. Loads running through an inverter are divided by 0.90 first, because AC conversion is not free.
2. Solar array
Array watts = daily Wh ÷ (peak sun hours × 0.75)
Peak sun hours are the daily hours equivalent to full-strength sun for a given location. The 0.75 is a real-world derate. Panels are rated under Standard Test Conditions of 1,000 W/m² at 25 °C cell temperature, which a roof in service almost never sees, so rated output overstates what you get.
3. Battery bank
Ah = (daily Wh × days of autonomy) ÷ (system volts × depth of discharge)
Depth of discharge is 0.80 for LiFePO4 and 0.50 for AGM and flooded lead-acid. This single difference is why a lithium bank roughly half the rated size of a lead-acid one delivers comparable usable energy.
4. Charge controller
Controller amps = (array watts ÷ system volts) × 1.25, rounded up to a standard size.
The 1.25 margin exists because photovoltaic output rises as cell temperature falls. A clear, cold morning can briefly push an array above its rating.
5. Cable and fuse
Two independent requirements are calculated and the thicker result governs:
- Ampacity. The conductor must safely carry the fuse rating, using the ABYC E-11 105 °C table. The fuse, not the running current, sets this, because the fuse decides the maximum current that can ever flow.
- Voltage drop. Held to 3% over the round-trip length. At 12V that is only 0.36V of headroom, which is why long runs need thick cable even at modest current.
Fuses protect the conductor, not the appliance, and belong within a few inches of the battery positive terminal.
Constants used
| Constant | Value | Why |
|---|---|---|
| Solar system derate | 0.75 | Covers wiring loss, controller efficiency, cell temperature, soiling and imperfect panel angle. Applied to the array output before sizing. |
| Peak sun hours | location-specific | Daily hours equivalent to 1,000 W/m² irradiance. Annual and winter figures are held per region. |
| LiFePO4 depth of discharge | 0.80 | Usable share of rated capacity in regular cycling. Deeper discharge is possible but shortens life. |
| AGM / lead-acid depth of discharge | 0.50 | Lead chemistries degrade sharply below roughly half capacity. |
| Cold-weather derate | 0.80 | Applied on request. Capacity falls in cold, and LiFePO4 must not be charged below 0 °C without a heater. |
| Inverter efficiency | 0.90 | DC-to-AC conversion loss. An AC load draws about 11% more from the battery than its label. |
| Controller & fuse margin | 1.25 | Standard 125% continuous-duty margin. Also covers cold-weather PV output above rating. |
| Voltage drop limit | 3% | Measured on the round-trip conductor length, not one way. |
| Wire ampacity basis | ABYC E-11, 105 °C | The marine/vehicle DC standard. Not the household NEC table, which gives thinner results. |
Sources
- ABYC E-11, AC and DC Electrical Systems on Boats — the conductor ampacity and overcurrent-protection basis used for all DC cable sizing here. American Boat & Yacht Council.
- NREL PVWatts and the National Solar Radiation Database — the reference for peak sun hours by location. pvwatts.nrel.gov.
- US Department of Energy, solar performance and efficiency — background on Standard Test Conditions and why rated output differs from delivered output. energy.gov.
- Manufacturer datasheets — cycle life and depth-of-discharge figures for LiFePO4 and AGM are drawn from published specifications rather than forum consensus.
What this method does not do
It sizes a system on paper. It does not account for a specific vehicle's existing wiring, shading from a roof rack, an unusual duty cycle, or an installation that departs from the assumptions above. Cost figures are a planning range, not a quote.
Most importantly, it is not a substitute for a qualified installer reviewing a high-current DC build. A van electrical system moves large currents through a vehicle, where an undersized conductor or a loose lug causes a fire rather than an inconvenience.
Corrections
Reported errors in a formula, constant or table value are checked against the source standard and fixed. Corrections are recorded in the project changelog. The method is maintained by Frank Zale; see the editorial policy for how content is produced and how commercial relationships are handled.
You can apply all of this yourself with the calculators, or read the reasoning behind each figure on the about page.