Battery Storage Sizing Calculator
How many batteries it takes to run your house through an outage — and how long one will actually last.
Estimate only. These are planning figures, not a quote, a firm price or an electrical design.
Assumptions used
- Electricity used per day
- 28 kWh
- Backup you want
- 24 hours
- Share of each battery left out of the estimate
- 20%, recommended
- Batteries
- 1
Based on a typical Santa Cruz County house — enter your own daily use for a closer number.
Runtime assumes your home draws 15% more than its average, so the number still holds on a heavier-than-usual day.
Runtime counts only 80% of each battery. The other 20% is a storm reserve: headroom for an outage that runs longer than planned, a cold night, or a battery that has lost some capacity with age.
The working
- Capacity we would aim for, reserve included52.5 kWh
- Average draw1.17 kW
- Derated draw used for runtime1.34 kW
By battery
Tesla Powerwall 3
- Usable per battery13.5 kWh
- Storm reserve2.7 kWh (20%)
- Counted on for runtime10.8 kWh
- Runtime, one battery8.0 hours
- Runtime, your batteries8.0 hours
- Batteries to hit your target3
- Batteries to hit the capacity target4
- Warranty10 years
FranklinWH aPower 2
- Usable per battery15 kWh
- Storm reserve3 kWh (20%)
- Counted on for runtime12 kWh
- Runtime, one battery8.9 hours
- Runtime, your batteries8.9 hours
- Batteries to hit your target3
- Batteries to hit the capacity target4
- Warranty15 years
Enphase IQ Battery 10C
- Usable per battery10 kWh
- Storm reserve2 kWh (20%)
- Counted on for runtime8 kWh
- Runtime, one battery6.0 hours
- Runtime, your batteries6.0 hours
- Batteries to hit your target5
- Batteries to hit the capacity target6
- Warranty15 years
Generac PWRcell
- Usable per battery14.3 kWh
- Storm reserve2.9 kWh (20%)
- Counted on for runtime11.4 kWh
- Runtime, one battery8.5 hours
- Runtime, your batteries8.5 hours
- Batteries to hit your target3
- Batteries to hit the capacity target4
- Warranty10 years
Energy and runtime figures are rounded to one decimal place at most; the battery counts use the exact values.
The link carries the figures you entered, usage included, and no name or address. They sit in the link itself, which your browser does not send to our servers.
What's in this number
Runtime is a battery's usable energy — its rated size times its depth of discharge, or the usable figure its maker publishes — less the storm reserve you choose (20% unless you pick another), which the estimate leaves uncounted, divided by your average hourly draw raised 15% so the number still holds on a heavier-than-usual day. The reserve is planning headroom for an outage that runs longer than expected, a cold night, or a battery that has lost some capacity with age. 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, daytime exports earn about a nickel per kilowatt-hour as a round planning figure; PG&E’s 2026-application table averages about 4 cents during 10am–4pm before bonus credits. Rates vary by hour, and using solar at home can avoid higher retail purchases. This tool shows sizing only — install costs come from a real consultation.
What's next?
About this estimate
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.
To confirm these figures for your home
Call Sunrise at (831) 786-5506 or visit sunriseelectricalcontracting.com