Home Battery Backup Runtime Calculator
Enter the battery's capacity and the average load you'll keep on during an outage, and get the usable kWh, the hours it lasts, and how fast central AC drains it.
Home battery runtime
D04Scenario A
Fridge 0.15 · lights 0.1 · router 0.02 · furnace fan 0.5 · well pump 1.0 · central AC 3.5 kW.
Estimated runtime
With central air on as well: 2.6 hours.
- Usable kWh
- 12.2
- Runtime
- 10.1 h
- With central AC running (+3.5 kW)
- 2.6 h
- Days at 10 kWh a day
- 1.2
Estimate. Formula and sources are under “How it works”. Change any input; the result updates as you type.
2026 note: The federal 25C and 25D tax credits ended for installations after Dec 31, 2025. Our numbers use 2026 rules. State rebates, HEAR/HOMES programs and utility programs may still apply.
Formula
How it works
- Usable kWh = capacity × depth of discharge × 0.90 (inverter and round-trip losses)
- Runtime (hours) = usable kWh ÷ average load in kW
- With central AC = usable kWh ÷ (average load + 3.5 kW)
- Days = usable kWh ÷ 10 kWh a day
This is how long the stored energy lasts with no solar recharging. With panels on the roof, a sunny day refills some or all of it. The runtime is an estimate: loads cycle on and off, so enter the average, not the peak.
Sources: Powerwall 3 (13.5 kWh, 11.5 kW continuous) and Enphase 5P (5 kWh) manufacturer specifications; ~90% round-trip efficiency; appliance load checklist from typical draw values.
Worked example
A worked example
Two 13.5 kWh batteries give 27 kWh of capacity, or 24.3 usable kWh after losses. At a 2.5 kW average load that's 9.7 h. Turn on a 3.5 kW central AC as well and it drops to 4.0 h. At 10 kWh a day, the stored energy covers 2.4 days.
Questions
FAQ
How long will a Powerwall run my house?
One 13.5 kWh battery gives about 12.2 usable kWh. At a 1.2 kW average load that's 10.1 hours; with only the refrigerator, lights, router and furnace fan on (0.77 kW from the checklist) it's about 15.8 hours.
Can a battery run central AC?
It can, but not for long. The 13.5 kWh example lasts 2.6 hours with a 3.5 kW central AC running on top of a 1.2 kW load. Check that the battery's continuous power covers the AC's startup too.
How many batteries for whole-home backup?
Start with the kWh you need per day and the number of days. For 10 kWh a day over 2 days, the Battery Size for Outage calculator gives 22.2 kWh nominal, or 2 units of 13.5 kWh.
What load should I enter?
Add up what you'd actually keep on with the checklist. A refrigerator, lights, a router and a furnace fan come to 0.77 kW; a well pump adds 1.0 kW while it runs.
Does depth of discharge matter?
Yes. At 90% depth, a 10 kWh battery gives 8.1 usable kWh, enough for 10.1 hours at 0.8 kW.
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