Battery Bank Size Calculator (Ah for Backup Time)

How to use the battery bank size calculator

Enter the load power, the backup time, the bank voltage, the inverter efficiency, the allowed depth of discharge, a design margin and the voltage of one battery. The calculator returns the energy the load uses, the amp-hour capacity the bank must have and how many batteries to connect in series. Choose the next standard Ah size above the result.

Battery sizing formula

Ah = (P × t) ÷ (V_bank × η × DoD) × k  ·  N_series = V_bank ÷ V_battery

Example

A 500 W load must run for 4 hours from a 48 V lead-acid bank through a 90% efficient inverter, discharging to 80% with a 1.25 ageing margin:

• Energy = 500 × 4 = 2,000 Wh
• Ah = 2000 ÷ (48 × 0.9 × 0.8) × 1.25 = 72.3 Ah
• Series batteries = 48 ÷ 12 = 4 → four 12 V, 75 Ah (or larger) batteries

Capacity needed on a 48 V bank

90% inverter efficiency, 80% depth of discharge, 1.25 margin:

Practical notes

• Lead-acid batteries last much longer when discharged to only 50%; LiFePO₄ tolerates 80–90% routinely.
• At 0–10 °C, lead-acid capacity falls by 10–20% — add a temperature correction factor.
• The charger must be able to recharge the bank in a reasonable time; see the battery charge time calculator.
• For the UPS kVA rating itself, use the UPS sizing calculator.

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