Battery Atlas

Sizing & runtime

Solar Battery Storage Calculator

Size the battery bank of an off-grid or backup solar system from daily energy use, days of autonomy, usable depth of discharge and efficiency.

Results are mathematical estimates from the values you enter — not measurements and not data about a specific product or vehicle.

d

%

Use the value the battery datasheet ties to its cycle-life rating.

%

Inverter and wiring losses. 100% for DC loads fed directly.

%

V

How does the Solar Battery Storage Calculator work?

Required nominal storage equals daily energy × days of autonomy, divided by the usable depth of discharge, the discharge efficiency and the temperature factor. Dividing by the system voltage gives amp-hours; the series count is system voltage ÷ battery voltage.

Formula

E(nominal) = E(day) × days ÷ (DoD × η × f(temp))
Ah(bank) = E(nominal) ÷ V(system)
S = V(system) ÷ V(battery)   (whole number)
P = ⌈Ah(bank) ÷ Ah(battery)⌉,  N = S × P

Worked example

3,000 Wh/day, 1 day, 48 V, DoD 80%, η 90%, 100% → 4,167 Wh nominal → 86.8 Ah @ 48 V → 4S1P of 12 V 100 Ah (4 batteries).

Limitations and warnings

  • This sizes storage only. Solar array and charge-controller sizing depend on location (irradiation) and are not covered.
  • Mixing batteries of different age or capacity in one bank shortens its life.

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.