Battery Bank Calculator

Battery bank capacity (Ah) = (Daily energy use in Wh x Days of autonomy) / (Battery voltage x Depth of discharge). For example, 3,000 Wh/day for 2 days, at 24V with 50% DoD: (3,000 x 2) / (24 x 0.50) = 500 Ah. Add a safety factor of 1.2 for aging and inefficiency.

Depth of discharge is the percentage of battery capacity used before recharging. Lead-acid batteries should not be discharged below 50% DoD to preserve cycle life (500-1,000 cycles at 50% DoD). Lithium iron phosphate (LiFePO4) batteries can be discharged to 80-90% DoD with 3,000-5,000 cycles at 80% DoD.

Days of autonomy is the number of days the battery bank can power the load without any charging (no sun or generator). Residential off-grid systems typically use 2-5 days autonomy. Grid-tied backup systems may only need 8-24 hours of backup for critical loads. More autonomy requires more battery capacity and cost.

Common battery bank voltages: 12V (small systems, RVs), 24V (mid-size residential, up to about 3 kW), 48V (larger residential and commercial systems, 3 kW and above). Higher voltage systems reduce wire current and losses. Most modern lithium battery systems and string inverters operate at 48V or higher.

Connect batteries in series to increase voltage: series batteries = Bank voltage / Battery voltage. Connect batteries in parallel to increase capacity: parallel strings = Total Ah required / Ah per battery. Total batteries = Series count x Parallel strings. Always use identical batteries from the same batch for parallel strings.