Battery Bank Sizing Calculator

Start with the energy your load uses over the autonomy window, which is daily watt-hours times the number of days. Divide that by the round-trip efficiency and again by the depth-of-discharge fraction to get the nominal (nameplate) watt-hours the bank must hold. Then divide nominal watt-hours by the system voltage to get amp-hours. The formula is: Ah = (daily Wh times days) divided by (system V times DoD times efficiency).

Depth of discharge (DoD) is the share of a battery's nominal capacity you actually use before recharging. Lead-acid banks are usually limited to about 50 percent to protect cycle life, while lithium (LiFePO4) often allows 80 to 100 percent. A lower DoD limit means you need a physically larger bank to deliver the same usable energy, because you reserve the rest.

Round-trip efficiency is the fraction of energy that survives a full charge then discharge cycle, including inverter and battery losses. If efficiency is 85 percent, you must pull more out of the bank than reaches your load, so the bank has to be larger. Lead-acid systems run roughly 80 to 85 percent round-trip, and lithium systems often reach 90 to 95 percent.

System voltage sets how watt-hours convert to amp-hours, since watt-hours equal volts times amp-hours. The energy and kWh requirement stay the same across voltages, but a higher bus voltage cuts the amp-hours and the current for a given load. That is why larger off-grid systems use 24V or 48V: lower current means smaller cables and lower losses.

It divides the required nominal amp-hours by the amp-hour rating of one battery and rounds up, since you cannot install a fraction of a battery. This assumes the batteries are wired to reach your system voltage, with series strings hitting the bus voltage and parallel strings adding capacity. The installed amp-hours and kWh shown reflect that rounded-up whole count.

Treat it as a planning estimate, not a final spec. Real capacity falls in cold temperatures and as batteries age, and lead-acid amp-hour ratings drop at higher discharge rates because of the Peukert effect. Size with margin and confirm against your specific battery, charge controller, and inverter manufacturer specifications before buying.