How Many Batteries Do You Need to Power a House in an Outage?
Count energy and power separately. The right formula (and the common one that double-counts), worked through for an average 28 kWh-a-day US home.
Updated October 27, 2026 · By the OutageWatt team · How we calculate
In short
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The formula, done right
Energy: batteries = daily kWh × (backup hours ÷ 24) ÷ 0.90 efficiency ÷ usable kWh per battery. Power: batteries = the most you'll run at once (kW) ÷ continuous kW per battery, and the biggest motor must start. You need the larger number.
A common online formula also divides by a 90% depth of discharge. Don't, when the spec is already in usable kWh, as it is for Powerwall 3 (13.5 kWh), IQ Battery 5P (5 kWh) and aPower 2 (15 kWh): you'd be paying for about 11% more battery than you need.
Average US home, worked through
| Battery | Half the house, 24 h | Whole house, 24 h | Whole house, 72 h |
|---|---|---|---|
| Tesla Powerwall 3 | 2 | 3 | 8 (over max) |
| Enphase IQ Battery 5P | 4 | 7 | 19 (over max) |
| FranklinWH aPower 2 | 2 | 3 | 7 |
Energy only, 28.4 kWh/day, 90% efficiency, no solar. Power and start-up checks can raise the count for homes with central AC or electric heat.
Why almost nobody backs up everything
Whole-home backup for three days without solar takes a garage full of batteries. Most people back up a sub-panel of essentials (fridge, furnace blower, well or sump pump, lights, internet) and let the dryer, range and water heater wait. That cuts the battery count by two thirds and is exactly what the planner helps you choose.
Frequently asked questions
How many Powerwalls do I need to power my house for a day?
For an average US home using about 28.4 kWh a day, about 3 Powerwall 3 units cover 24 hours of everything with no solar. Backing up only essentials, one is usually enough for several days.
What's the formula?
Batteries for energy = daily kWh × (backup hours ÷ 24) ÷ efficiency ÷ usable kWh per battery. Batteries for power = your peak kW ÷ continuous kW per battery. You need the larger of the two.
Should I apply a depth-of-discharge factor?
Only if the battery's spec gives total (nominal) capacity. Powerwall 3, IQ 5P and aPower 2 are quoted in usable kWh already, so applying a 90% DoD on top double-counts and oversizes the system.
Free tool
Size it for your circuits
The planner counts energy, power and start-up surge for you.
OutageWatt is independent and not affiliated with any brand named here. Specs come from makers' spec sheets and are dated; anything wired into your home needs a licensed electrician and a transfer switch or interlock. Never backfeed through an outlet.