The runtime formula
Runtime hours equals usable kWh times inverter efficiency times 1000, divided by load watts. Shaving load is the cheapest way to add hours.
What to back up
Refrigerator, furnace blower, sump pump, lights, phones, and medical devices. Add their watts; skip HVAC compressors and dryers unless the bank is large.
Stretching through multi-day outages
Pair the bank with solar for daytime recharge, or keep a generator for the last resort. Size autonomy days, not just hours.
Formulas used
Runtime
usable kWh x efficiency x 1000 / load watts
Worked examples
10 kWh bank, 1 kW load
10 x 0.9 x 1000 / 1000 = 9 hours.
Common questions
How long will a 10 kWh battery last?
About 9 hours at a 1 kW average load after inverter losses; halving the load doubles the time.
Can a battery run central AC?
Only briefly: a 3-4 kW compressor drains a 13.5 kWh bank in about 3 hours.
Sources
- Energy Saver: Battery Storage U.S. Department of Energy
- Electricity Data U.S. Energy Information Administration