Battery Capacity Chart by Backup Need
How many kWh of battery backup your essentials need, by load and hours, with solar and generator pairing math.
Battery sizing starts from the loads you must keep running and for how long. This chart converts common backup goals into usable kWh, using a 90% depth of discharge and 85% round-trip efficiency for lithium.
| Backup goal | Load | Hours | Usable energy | Battery size |
|---|---|---|---|---|
| Fridge + internet + lights | ~250 W | 24 | 6 kWh | ~6.7 kWh battery |
| Essentials + furnace fan | ~800 W | 12 | 9.6 kWh | ~10.7 kWh |
| Essentials overnight | ~800 W | 16 | 12.8 kWh | ~14 kWh |
| Whole essentials day | ~1,200 W | 24 | 28.8 kWh | ~32 kWh |
| Solar self-consumption (10 kWh/day home) | — | — | — | 10–14 kWh typical |
Rule of thumb: battery kWh is roughly (watts × hours) ÷ 1,000 ÷ 0.77. Stack batteries to the number; most rack systems parallel cleanly.
A battery sized for overnight essentials pairs with a daytime solar array sized to recharge it: roughly battery kWh ÷ 4 peak sun hours × 1.2 = array kW. Hybrid inverters let a standby generator top up the battery without shutting the house down — see our standby generator sizing chart for the genset side and the transfer switch sizing chart for the switchover.
Lithium vs lead-acid for backup?
Lithium gives 90% usable depth and 4,000+ cycles; lead-acid gives 50% and 500–1,200. For daily cycling lithium wins; for rare outages either works.
Can a battery run central AC?
A 3-ton compressor at 3.5 kW running needs ~14 kWh for four hours — possible with a 16+ kWh battery and a soft start on the compressor.
Building backup power?
Batteries, hybrid inverters, and standby generators in stock — contractor pricing and system sizing from a real desk.
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