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Off-grid battery bank size calculator.
kWh and amp-hours for your autonomy days, and the series and parallel layout to build it.
Inputs
Battery bank
Enter your numbers.
How big should an off-grid battery bank be?
Battery kWh = daily kWh x days of autonomy / usable depth of discharge / efficiency. Amp-hours = battery watt-hours / system volts. Batteries in series add voltage (system volts / battery volts per string) and strings in parallel add amp-hours (needed Ah / battery Ah, rounded up). DOE's Small Wind Guidebook says off-grid banks are typically sized for 1 to 3 days, and Battery University notes manufacturers often rate batteries at 80% depth of discharge.
Worked example. 5 kWh per day, 2 days, 80% usable, 90% efficiency: 5 x 2 / 0.8 / 0.9 = 13.9 kWh, which is 289 Ah at 48 V. With 12 V 100 Ah batteries that is 4 in series and 3 strings in parallel, 12 batteries and 14.4 kWh.
Frequently asked questions
How many batteries do I need for off-grid?
Divide the amp-hours this tool gives by one battery's Ah for the parallel strings, and divide system voltage by battery voltage for the series count. The example above needs 12 batteries of 12 V 100 Ah.
How many days of autonomy should an off-grid battery bank have?
DOE says banks are typically sized for 1 to 3 days. More days cost more; a generator or wind turbine lets you use fewer.
What depth of discharge should I use?
Use the figure in your battery maker's datasheet. Battery University notes makers often rate at 80% DoD and that shallower cycling extends service life.
Why does a higher system voltage help?
The same watts need fewer amps at 48 V than at 12 V, so cables and fuses are smaller. Check cable size with a voltage drop tool.
Sources
Checked
- U.S. Department of Energy WINDExchange: Small Wind Guidebook
- Battery University: BU-501 Basics about Discharging
- U.S. EIA: Measuring electricity
The 1 to 3 day range is from DOE and 80% DoD rating practice is from Battery University. The efficiency and depth of discharge defaults are placeholders; use your own battery and inverter datasheets. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Planning estimate only. Large battery banks can deliver very high short-circuit current. Fuse and install them to your local electrical code, or hire a qualified installer.