Battery Bank Sizing

It tells you the battery capacity — in kWh and as a number of batteries — needed to run a chosen load for a target number of hours during an outage. Enter what you want to power and how long, and it sizes the bank for you.

First it finds the usable energy you need: load in kW × backup hours. Then it grosses that up for losses — dividing by the depth of discharge and the inverter efficiency — to get the rated (nameplate) battery capacity. For example, a 500 W load for 8 hours is 4 kWh usable; at 90% depth of discharge and 92% inverter efficiency that needs about 4.8 kWh of rated battery.

You never draw a battery to empty (that shortens its life), and the inverter loses some energy as heat converting DC to AC. So the nameplate capacity always has to be larger than the energy actually delivered to your appliances. Lower depth of discharge or a less efficient inverter means a bigger bank.

Enter the size of one battery (for example a 5 kWh module) and the calculator divides the total capacity needed by that, always rounding up to a whole battery. It also shows the total installed capacity once rounded.

Yes. This is the minimum for a best-case, steady load. Batteries fade with age and cold, and startup surges and forgotten loads add up. Round up to the next battery and keep some headroom rather than sizing to the exact figure. Remember you also need enough solar or grid charging to refill the bank between outages.