Off-Grid Battery Bank Capacity Calculator

Required watt-hours equal daily load times days of autonomy, divided by depth of discharge times efficiency. Required amp-hours are that figure divided by system voltage. So 4000 Wh over 1 day at 0.5 depth and 0.85 efficiency is 4000 / 0.425, which is 9411.76 Wh, then divided by 12 V gives 784.31 Ah.

You cannot safely use every watt-hour stored, and some energy is lost charging and inverting. Dividing by the usable depth of discharge and the round-trip efficiency inflates the raw bank so the portion you actually draw still meets the daily load.

Lead-acid banks last far longer when cycled to about 0.5, meaning half the rated capacity. Lithium iron phosphate can run to 0.8 or higher. Use the figure your battery maker recommends, and note the field accepts values above 0 up to 1.

Days of autonomy multiply the requirement directly. Two days of backup needs twice the bank of one day for the same daily load, as the second example shows at 18,823.53 Wh versus 9,411.76 Wh.

The stored watt-hours stay the same at any voltage. A higher system voltage simply means fewer amp-hours and thinner cable for the same energy, which is why the 48 V example needs only about 110 Ah while the 12 V one needs 784 Ah.

The default 0.85 covers typical inverter and charge losses for a mixed AC and DC system. A well-matched lithium and high-efficiency inverter setup can reach 0.90 to 0.95. Lower it if you run older equipment or long cable runs.

No. This sizes the storage only. Use a separate solar panel calculator to size the array and charge controller that refill the bank each day, then check the two results work together.

Daily load, days and voltage must be greater than 0, and depth of discharge and efficiency must be above 0 and at most 1. Anything outside those bounds shows a short hint instead of a result.

No. The calculation runs locally in your browser using plain arithmetic. None of your load figures leave your device, so the tool works offline once loaded.