Battery sizing starts with the AC energy you want to supply during an outage. Enter average daily consumption and a backup duration; the calculator spreads daily use evenly over 24 hours so a few backup hours are not mistaken for multiple full days of energy.
Formula and assumptions: backup AC Wh = daily kWh × 1,000 × backup hours ÷ 24, or daily kWh × 1,000 × backup days. Required nominal DC Wh = required AC Wh ÷ (usable fraction × inverter efficiency). Ah = nominal Wh ÷ system voltage. Battery quantity rounds up the nominal energy divided by one selected battery’s nominal Wh. This model assumes a fully charged battery and no solar recharge during backup.
Household example: 10kWh/day averaged across 24 hours needs 5kWh AC for 12 hours. At 90% usable capacity and 90% efficiency, nominal storage needs about 6.173kWh, or 120.56Ah at 51.2V. A 51.2V 100Ah unit contains 5.12kWh nominal energy; the energy-only count rounds up to two units. This does not establish that those units can be connected together or can supply the required current.