Launch solar guide
Battery sizing for backup
Estimate usable energy, runtime, depth of discharge, and surge.
Author: SolarGabay EditorialReviewer: Solar Methodology TeamUpdated July 15, 2026
Summary answer
Battery size depends on the energy needed during an outage, maximum simultaneous power, motor surge, allowed depth of discharge, conversion losses, and reserve. Start by limiting backup to critical loads.
What changes the answer
- Energy capacity determines runtime; inverter power and surge determine which loads can start.
- Battery chemistry and control settings change usable capacity and expected life.
- Air conditioners, pumps, refrigerators, and motors require surge allowances.
- Temperature, aging, standby losses, and cloudy-weather operation reduce practical runtime.
Practical checklist
- List critical loads with running watts, surge watts, quantity, and required hours.
- Decide which loads cannot run simultaneously during an outage.
- Confirm usable—not only nominal—battery capacity.
- Request a commissioning test using actual backup circuits.
Evidence and limitations
The battery calculator provides an early range. Final battery, inverter, protection, and wiring design must be completed by a qualified provider.
Source transparency
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