Home Battery Backup Runtime Calculator
Estimate how long a home battery (Powerwall-style) will run your house or a critical-loads panel during an outage, from usable capacity and your load.
How it works
Deliverable kWh = capacity × (1 − reserve%) × efficiency% · Runtime h = deliverable ÷ load kW
A battery only gives you the energy above its reserve floor, and some is lost turning DC storage into AC for your outlets, so start from usable capacity, subtract the reserve, and multiply by round-trip efficiency. Divide that by your average draw in kilowatts and you have hours. Runtime is dominated by the load: dropping from whole-home to a critical-loads panel (fridge, lights, network) can turn a couple of hours into most of a day.
Worked example
A 13.5 kWh battery kept 10% in reserve at 90% efficiency delivers 13.5 × 0.9 × 0.9 = 10.9 kWh. Running a 1.2 kW essentials load, that lasts 10.9 ÷ 1.2 ≈ 9.1 hours.
Frequently asked questions
Why use usable capacity, not nameplate?
Batteries reserve part of their nameplate capacity to protect cell life, so the usable figure is what you can actually draw. Most spec sheets list a usable kWh separately — use that number here.
How do I estimate my outage load?
Add up only what you keep running: a fridge (~0.15 kW average), LED lights, Wi-Fi, and phone charging often total 0.5–1.5 kW. Air conditioning, electric heat, ovens and well pumps are large and will shorten runtime dramatically.
Can solar extend this?
Yes — if you have solar and it keeps producing during a daytime outage, it recharges the battery and can stretch runtime indefinitely as long as production meets your load. This calculator assumes battery-only, the worst case.
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This calculator is for educational purposes. Double-check important results before acting on them.