This battery size calculator works out the battery capacity you actually need, in amp-hours and kilowatt-hours, to run a given load for a set time. It is the sizing tool you reach for when planning an off-grid solar bank, a caravan or boat house battery, a backup supply or an inverter setup, where the question is not how long a battery you already own will last but how big a battery to buy in the first place. You enter the load in watts, how many hours you want it to run, the system voltage (12, 24 or 48 volts are common), the usable depth of discharge for your battery chemistry, and an allowance for inverter and wiring losses. The calculator turns your load and runtime into watt-hours, then divides by the depth of discharge and the efficiency so the battery is large enough to deliver that energy without being flattened. Because lead-acid cells should only give up about half their rated capacity while lithium can safely give 80 to 90 percent, the depth of discharge setting has a big effect on the answer. Treat the result as a starting point, size up for cold weather or growth, and check the maker's specifications before you buy.
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W
hours
V
%
%
471 Ah
recommended battery capacity
Energy per cycle2,400 Wh
Battery bank5.65 kWh
System voltage12 V
Sized for 600 W for 4 hours (2,400 Wh). After a 50% depth of discharge and 85% system efficiency, you need about 471 Ah at 12 V, roughly a 5.65 kWh bank.
Add headroom for cold weather, ageing and future loads. Depth of discharge depends on chemistry: about 50% for lead-acid, 80 to 90% for lithium. Estimate only; check the maker's specifications.
How it works
The energy you draw each cycle is the load in watts times the runtime in hours, giving watt-hours. A battery cannot be flattened fully and the inverter and wiring lose some energy, so the nominal battery must be larger. The capacity in amp-hours is the watt-hours divided by the system voltage, then divided again by the usable depth of discharge and the system efficiency. The battery bank in kilowatt-hours is the same nominal energy expressed in kWh. Lower the depth of discharge and the required capacity rises, which is the trade you make to extend battery life.
Worked example
You want to run a 600 W load for 4 hours from a 12 V bank. That is 600 by 4, which is 2,400 watt-hours drawn per cycle. Using half the capacity (50% depth of discharge) and allowing 85% for inverter and wiring losses, the usable fraction is 0.5 by 0.85, which is 0.425. So the nominal energy needed is 2,400 divided by 0.425, about 5,647 watt-hours, or 5.65 kWh. Divided by 12 volts that is about 471 amp-hours. Switch to lithium at 80% depth of discharge and the same load needs only about 294 amp-hours.
