Battery Storage Sizing Calculator
How many batteries it takes to run your house through an outage — and how long one will actually last.
Based on a typical Santa Cruz County house — enter your own daily use for a closer number.
Runtime uses a derated draw so the number still holds on a heavier-than-usual day.
The working
- Capacity we would aim for42 kWh
- Average draw1.17 kW
- Derated draw used for runtime1.34 kW
By battery
Tesla Powerwall 3
- Usable per battery12.8 kWh
- Runtime, one battery9.6 hours
- Runtime, your batteries9.6 hours
- Batteries to hit your target3
- Batteries to hit the capacity target4
- Warranty10 years
FranklinWH aPower 2
- Usable per battery12.9 kWh
- Runtime, one battery9.6 hours
- Runtime, your batteries9.6 hours
- Batteries to hit your target3
- Batteries to hit the capacity target4
- Warranty12 years
Enphase IQ Battery 10C
- Usable per battery10.1 kWh
- Runtime, one battery7.5 hours
- Runtime, your batteries7.5 hours
- Batteries to hit your target4
- Batteries to hit the capacity target5
- Warranty15 years
Generac PWRcell
- Usable per battery14.3 kWh
- Runtime, one battery10.6 hours
- Runtime, your batteries10.6 hours
- Batteries to hit your target3
- Batteries to hit the capacity target3
- Warranty10 years
What's in this number
Runtime is usable capacity — a battery's rated size times its depth of discharge — divided by your average hourly draw, with a safety margin so the number holds on a heavier-than-usual day. Since the federal residential clean energy credit expired at the end of 2025, the case for storage now rests on self-consumption and backup rather than a tax credit: under NEM 3.0 the power you'd export earns about a nickel a kilowatt-hour, so using it yourself is worth far more than selling it. This tool shows sizing only — install costs come from a real consultation.
What's next?
Sunrise Electrical Contracting holds California CSLB licence #C10-1097237 (C-10, Electrical).
This is a planning number, not a substitute for a licensed electrician confirming the panel, the service and the utility feed in person.
We work in Santa Cruz County.