Battery Atlas

Vehicles, trucks & marine

RV & Marine House Battery Calculator

Size the house (service) bank of a motorhome, camper or boat from daily consumption, days off-grid and chemistry. Optional solar replenishment.

Use the Electrical Consumption calculator if unsure.

d

%

100% at ~25 °C. Lower it for cold operation per the battery datasheet.

%

W

h

%

Formula

DoD: flooded/AGM/GEL 50% · LiFePO4 80% (default assumption)
Ah(bank) = Ah/day × days ÷ DoD ÷ f(temp) × (1 + margin)
Solar Ah/day = W × sun h × η ÷ V

Worked example

80 Ah/day, 2 days, AGM (50%), 100% temp, 20% margin → 384 Ah → 4 × 100 Ah in parallel (12 V).

Limitations and warnings

  • Keep the engine-starting battery separate from the house bank (isolator or DC-DC charger).
  • Boats: follow ABYC (Americas) or ISO 10133 / local rules for installation.

Four values that are often confused

Nominal capacity (Ah)
Charge a battery delivers under rated conditions, usually over 20 hours (C20) at 25 °C.
Usable capacity
The part of the nominal capacity you can use without damaging the battery — about 50% for lead-acid, 80–90% for LiFePO4.
Cranking current (CCA)
Current deliverable for a short time in the cold to start an engine. It measures power, not stored charge, and cannot be converted to Ah.
Stored energy (Wh)
Ah × voltage. The right unit to compare batteries of different voltages.