Sizing & runtime

12 V, 24 V & 48 V Battery System Calculator

Size a battery bank for a 12, 24 or 48 V system and see how system voltage changes current.

System voltage

Average load

Hours per day

Days of autonomy

Usable depth of discharge

Voltage per battery

Capacity per battery

Formula

I = P ÷ V(system)
Ah(bank) = P × h × days ÷ (V × DoD)
S = V(system) ÷ V(battery),  P = ⌈Ah(bank) ÷ Ah(battery)⌉

500 W × 4 h, 1 day, 24 V, DoD 50%, 12 V 100 Ah batteries → 166.7 Ah bank → 2S2P = 4 batteries; current 20.8 A (41.7 A at 12 V).

Higher system voltage lowers current, cable size and losses, but all equipment must be rated for it.

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.