Wire Your Rig

Solar & DC-DC charging calculator

See whether solar plus driving puts back what you use each day, and how long the bank lasts if it doesn't.

Enter values above.

The spreadsheet adds: a Charging tab that pulls daily use and bank size straight from the Load Budget and Battery Sizing tabs (solar, DC-DC and shore power). Free vs. spreadsheet

Safety: this is a planning estimate, not an engineering sign-off. DC systems can start fires and lithium batteries can deliver very high fault current. Check every result against the manufacturer's instructions and current ABYC/NEC or local rules, put a correctly rated fuse or breaker close to the battery on every positive cable, and have a qualified installer check work you are not sure about. We are not responsible for how results are used.

How this is calculated

Solar. Daily Wh = panel watts × peak sun hours × real-world factor; Ah = Wh ÷ system volts. "Peak sun hours" are equivalent hours of 1,000 W/m² sun, and they change a lot with location and season, so use NREL PVWatts for your area and the worst month you plan to camp. The real-world factor (75% default) is a planning assumption covering heat, panel angle, dirt, wiring and controller losses. Flat-mounted roof panels often do worse.

DC-DC. Ah = charger amps × hours driven × average output fraction. The charger's amp rating is on the battery side, and output drops as a lithium bank nears full.

Balance. Net = solar + DC-DC + other − daily use. If it's negative, days until the usable limit = bank Ah × usable % ÷ daily deficit. Solar needed to cover use alone = daily Ah × volts ÷ (sun hours × factor).

Planning a whole build?

The paid spreadsheets run these same tested formulas for a whole build in one saved file. Compare free vs. spreadsheet.

12V Load & Wire Gauge Calculator spreadsheet on Etsy, $9.00

Overland Rig Planning Bundle (12V + Payload + Trip spreadsheets) on Etsy, $18.99

Sources

  1. NREL PVWatts calculator (look up peak sun hours for your location and month). pvwatts.nrel.gov
  2. Victron Energy: Wiring Unlimited (cable, fuse and inverter wiring guidance). www.victronenergy.com
  3. Victron Energy: Lithium-ion vs AGM batteries (usable depth of discharge). www.victronenergy.com