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Battery Backup Runtime Calculator
See how long a Tesla Powerwall 3, Enphase IQ Battery 5P, or FranklinWH aPower 2 would keep your home running in an outage — based on the appliances you'd actually keep on.
The short answer
Runtime = stored energy ÷ what you keep running
A home battery stores a fixed number of kilowatt-hours. Divide that by the wattage of the loads you keep on and you get hours of backup. Essentials-only backup can last a day or more; adding central AC or EV charging can cut that to a few hours.
Two specs decide everything. Stored energy (kWh) sets how long the battery lasts, and continuous output (kW) sets how much can run at the same time. Here's how the three batteries we install most compare, per each manufacturer's published specifications:
| Battery | Stored energy | Continuous output | Spec source |
|---|---|---|---|
| Tesla Powerwall 3 | 13.5 kWh | 11.5 kW | Tesla Powerwall 3 Datasheet (energylibrary.tesla.com) (accessed 2026-07-22) |
| Enphase IQ Battery 5P | 5 kWh | 3.84 kW | Enphase IQ Battery 5P product specifications (enphase.com) (accessed 2026-07-22) |
| FranklinWH aPower 2 | 15 kWh | 10 kW | FranklinWH aPower 2 product page (franklinwh.com) (accessed 2026-07-22) |
Manufacturer specifications for a single unit; all three systems stack additional units for more capacity and output. Deciding between them? See our battery storage service page for how backup design works, or the Tesla Powerwall 3 page for a deep dive on that product.
Interactive estimate
Try it with your own appliances
Pick a battery and toggle the loads you'd keep on. Everything updates instantly — and every number is an estimate, not a quote.
Estimate your backup runtime
Pick a battery, then choose what you'd keep running. No email required.
Backup profile
Wattages are U.S. DOE sample figures (sources below) — your appliances may differ. All loads are assumed to run continuously while selected, except the refrigerator, which cycles on and off (DOE estimates it runs at max wattage about a third of the time).
Your estimate
about 35.4 hours
Estimated backup runtime for a single Tesla Powerwall 3 (13.5 kWh) powering your selection — an estimate, not a guarantee.
Average draw
381 W
what drains the battery each hour
Peak draw
531 W
if everything ran at once
Where the numbers come from
Assumptions and sources
No black box: the tool divides the battery's published stored energy by the combined wattage of your selected loads. Wattages are U.S. Department of Energy sample figures — your appliances may differ.
Appliance wattages used
- Refrigerator: 225 W running (DOE Energy Saver Appliance Energy Calculator); cycles on/off — DOE: runs at max wattage about 1/3 of the time plugged in.
- LED lights (about 10 bulbs): ≈15 W per 60-W-equivalent LED bulb × 10 — derived from DOE Energy Saver: residential LEDs use at least 75% less energy than incandescent (60 W × 25% = 15 W).
- Wi-Fi router / modem: 6 W (DOE Energy Saver Appliance Energy Calculator, “Router/DSL/cable modem”).
- Television (LCD): 150 W (DOE Energy Saver Appliance Energy Calculator, “Television, LCD”).
- Microwave oven: 1,500 W while cooking (DOE Energy Saver Appliance Energy Calculator).
- Central air conditioner (3-ton): ≈3,100 W — 36,000 Btu/h (3-ton) ÷ EER2 11.7, the federal minimum efficiency for split-system ACs under 45,000 Btu/h installed in the Southwest region incl. California (10 CFR 430.32(c)(6)).
- EV charger (Level 2): 7,200 W — most residential Level 2 chargers operate at up to 30 A / 7.2 kW (U.S. DOE Alternative Fuels Data Center).
Primary sources
- U.S. DOE Energy Saver, Appliance Energy Calculator — sample wattages (refrigerator 225 W; router/modem 6 W; LCD TV 150 W; microwave 1,500 W). Page removed in an energy.gov restructure; verified via the Internet Archive snapshot of Feb 4, 2024. Accessed July 22, 2026.
- U.S. DOE Energy Saver, “Estimating Appliance and Home Electronic Energy Use” — refrigerator duty-cycle rule (runs at max wattage ~1/3 of the time plugged in). Verified via the Internet Archive snapshot of June 1, 2025. Accessed July 22, 2026.
- U.S. DOE Energy Saver, “LED Lighting” — residential LEDs use at least 75% less energy than incandescent (basis for ~15 W per 60-W-equivalent bulb). Verified via the Internet Archive snapshot of Apr 1, 2025. Accessed July 22, 2026.
- 10 CFR 430.32(c)(6) — federal minimum EER2 of 11.7 for split-system central ACs under 45,000 Btu/h installed in the Southwest region (incl. California); basis for the ~3,100 W (3-ton) figure. Accessed July 22, 2026.
- U.S. DOE Alternative Fuels Data Center, “Electric Vehicle Charging Stations” — most residential Level 2 chargers operate at up to 30 A, delivering 7.2 kW. Accessed July 22, 2026.
Estimates assume no solar recharging during the outage and that selected loads run continuously (except the refrigerator, which cycles). Real-world runtime varies with weather, temperature, state of charge, and usage — treat every figure as an estimate, never a guarantee.
Backup runtime FAQ
Common questions
How long will a home battery last during a power outage?
It depends on how much energy the battery stores and what you run. Backing up just the essentials — refrigerator, LED lights, Wi-Fi, and a TV — a 13.5 kWh battery like the Tesla Powerwall 3 runs for roughly a day and a half in our estimate, while a 5 kWh Enphase IQ Battery 5P covers the same loads for around half a day. Add central AC or EV charging and runtime drops to a few hours. Use the calculator above for your own mix — and remember it's an estimate, not a guarantee.
Can a home battery run my air conditioner?
Often, but it's the fastest way to drain one. Two limits matter: continuous power (a 3-ton AC drawing ~3.1 kW fits within a Powerwall 3's 11.5 kW or an aPower 2's 10 kW output, but exceeds a single IQ Battery 5P's 3.84 kW) and stored energy (running that AC continuously uses ~3.1 kWh every hour). Most homeowners back up AC only with multiple battery units or solar recharging the battery during the day.
Does solar recharge the battery during an outage?
Yes — if you have solar panels and a battery system configured for backup, the panels recharge the battery whenever the sun is up, which can stretch backup from hours into days. That's why the calculator's figure is conservative: it assumes no recharging at all.
What's the difference between essentials and whole-home backup?
Essentials backup wires a critical-loads panel — refrigerator, lights, internet, outlets — so a smaller battery lasts longer. Whole-home backup keeps everything available, including 240 V loads like AC and EV charging, which usually calls for a higher-output battery (or several stacked units). We design both configurations; the right answer depends on your panel, loads, and budget.
Get a backup design sized to your actual panel
The calculator is a starting point. Your real runtime depends on your loads, your electrical panel, and how many battery units make sense — we'll model it precisely and show you every option, free and no-pressure.