Home Backup Battery Size Calculator
Battery marketing quotes capacity in ways designed to confuse. This calculator works the way an installer (a good one) does: what loads do you need to run, for how long? It returns two numbers that define your purchase — required inverter output (watts) and required storage (kWh) — then shows which class of product fits.
1. What needs to stay on during an outage?
2. How long should it last?
How the math works
- Inverter watts = sum of running watts of everything you checked, plus the single largest motor surge (well pumps, AC compressors, and fridges briefly draw 2–3× their running watts at startup).
- Storage kWh = total daily energy of checked loads × days ÷ 0.85 (real-world inverter and conversion losses run ~10–20%). Solar recharge reduces the requirement by the factor you select.
- 240V loads (well pump, central AC, electric range/dryer) require a battery system with 240V split-phase output — many portables are 120V only. The result flags this automatically.
Appliance wattages below are typical US figures; your nameplate may differ. For deep-dives on specific loads, see how long a battery runs a refrigerator.
Next step
Take your two numbers to the whole-home battery rankings (10 kWh+ needs) or the portable power station guide (hours-to-a-day needs). On a budget decision between battery and generator? That comparison is here.