Calculator
Backup power sizing
Last reviewed November 2, 2026
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Your estimate
Energy for the outage
12.4 kWh
12.4 kWh per day
Battery needed
13.8 kWh
≈ 2 typical 13.5 kWh units
Power at once
3.2 kW
peak start-up; 1.6 kW running
A portable or standby generator of at least 4.5 kW would cover these loads with headroom. A battery system needs both enough energy (kWh) and enough output (kW) to start motors like sump pumps and fridges.
Generators must be connected through a transfer switch installed under an ESA permit — never backfeed through an outlet. Read our outage preparedness guide and backup power options.
Pairing a battery with rooftop solar lets it recharge during a multi-day outage. Estimate it for your roof
Assumptions and method
Wattages and daily run-hours are typical averages; motor loads cycle on and off. Battery size assumes 90% usable capacity and round-trip efficiency combined.
Peak power adds the largest single start-up surge to the total running load. Generator sizing adds 25% headroom.