Start from usable energy
List the loads the battery must carry and their daily kWh. Usable energy equals daily kWh times autonomy days. This is the number everything else derives from.
Convert usable to nominal with DoD
Divide usable kWh by depth of discharge: 0.9 for lithium, 0.5 for lead-acid. The result is the nominal bank size to shop for.
Count modules and check power
Divide nominal kWh by one module's usable kWh and round up. Then confirm the inverter's continuous kW covers your largest simultaneous load.
Formulas used
Usable energy
daily kWh x autonomy days
Nominal bank
usable kWh / DoD
Worked examples
30 kWh home, 1 day, lithium
30 / 0.9 = 33.3 kWh nominal; at 13.5 kWh per module, 3 modules.
Common questions
How many kWh does a house need?
Most US homes use 20-35 kWh per day; backing up essentials usually needs 10-20 kWh of usable storage.
How many batteries for whole-home backup?
Divide your daily kWh by DoD and by module size; 2-4 Powerwall-class modules cover most homes for one day.
Sources
- Electricity Data U.S. Energy Information Administration
- Energy Saver: Battery Storage U.S. Department of Energy