How to work out what you need to back up
Write down what must stay on in an outage. Usually that is the refrigerator, some lights, a modem and perhaps a sump pump or a furnace fan. Then work out an average, because a battery has to carry the load across the hours, and most of this cycles on and off. A refrigerator does not draw its label wattage all day long.
Your bill is the best reality check. Divide your monthly kilowatt hours by 30 for a daily figure, and you have what a day in your normal life costs in energy. Take what is essential out of that. Then think about how long an outage you are really planning for, because there is a big difference between a few hours and three days without sun for any solar panels.
Then add up what can run at the same time, in watts. That is your peak, and it is a separate check from the energy. Battery data sheets list a continuous output and a separate motor start, in locked rotor amps, and a compressor or pump can need far more for a moment than it does to run.
A worked example
Here is a house keeping 500 watts of essentials going for a full day, with up to 3,000 watts at once. These lines come from the same formula the calculator above runs.
500 W for 24 hours, 3,000 W peak
- Energy: 500 × 24 ÷ 1000 = 12 kWh
- Powerwall 3, for the energy: 1
- Powerwall 3, for the power: 1
- Enphase IQ 5P, for the energy: 3
- One Powerwall 3 lasts 27.0 hours at this load
Data sheets state these as usable AC energy, so no further division for depth of discharge or efficiency is needed. A generator is the usual alternative, and the generator size calculator starts from the same list. To see how much a panel array could add back during the day, try the solar panel calculator.
Batteries for common outages
These count energy only, at each maker's usable rating, with no recharge from solar. Add your own peak in the calculator above to check the power as well.
| Average load, for | Energy | Enphase IQ 5P | Tesla Powerwall 3 | FranklinWH aPower 2 |
|---|---|---|---|---|
| 300 W, 8 hours | 2.4 kWh | 1 | 1 | 1 |
| 300 W, 24 hours | 7.2 kWh | 2 | 1 | 1 |
| 500 W, 24 hours | 12.0 kWh | 3 | 1 | 1 |
| 1000 W, 24 hours | 24.0 kWh | 5 | 2 | 2 |
| 1000 W, 72 hours | 72.0 kWh | 15 | 6 | 5 |
| 1500 W, 24 hours | 36.0 kWh | 8 | 3 | 3 |
Mistakes that leave you in the dark
- kW for kWh. Kilowatts are how hard a battery can push. Kilowatt hours are how long it can keep going. You need both.
- Dividing twice. A data sheet's usable figure is already net of reserve. Taking off depth of discharge again oversizes the system.
- Using round trip for a discharge. The 89 to 96 percent covers charging as well as discharging, so it does not apply to a load on its own.
- Forgetting the big loads. Central air, an electric range or a dryer can drain a battery in hours. Keep them off the backup panel.
- Counting on the sun. The worst storms are often overcast for days, so plan with no recharge.
Questions
How big a battery do I need to get through an outage?
Multiply your average watts by the hours, and divide by 1,000. Essentials averaging 500 watts for 24 hours take 12 kilowatt hours, about one Powerwall 3 at 13.5 kWh. Check the peak power too, since a battery can have enough energy and still not start a motor.
What does depth of discharge mean?
It is the share of a battery's capacity you can draw before stopping. But the data sheets we read state usable energy, already net of that. Enphase's 5 kWh is usable, so dividing it again would count it twice. Use it only for a nominal rating.
Do I need to allow for efficiency?
Not on a usable AC rating, because that is what the battery delivers. Figures such as 89 to 96 percent are round trip, covering both charging and discharging, so applying them to a discharge overstates what you need.
How much battery backup for a sump pump?
Add its running watts to your load, but check the motor start as well. FranklinWH and Tesla list 185 A locked rotor amps, and a battery has to be able to start your pump as well as run it. Your pump's label has the figures.
Can a battery run my whole house?
Rarely for long. Central air, electric ranges and dryers use a lot, and a whole-house load can drain a battery in hours. Most people put essentials on a backup panel and keep those large loads off it.
When do I need a professional?
You need a licensed electrician or installer for any battery connected to your house, as it involves high-voltage wiring, a backup panel, permits and placement rules. Enphase's data sheet refers to 2021 IRC requirements for where a battery can go, so ask yours.
This battery backup figure is for planning, from data sheet specs that change and capacity that fades. Wiring one in has to meet the National Electrical Code your area has adopted, and a licensed electrician or installer should do it, since faulty wiring and battery placement can cause shock or fire. Local rules can limit where a battery goes, so ask before you buy.
Sources and review
- IQ Battery 5P data sheet, Enphase, 2025.
- aPower 2 data sheet, FranklinWH, 2026.
- Powerwall 3 datasheet, Tesla, search excerpt only, read November 2026.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Energy
kWh = Average watts × Hours ÷ 1000
- Average watts
- Your load averaged over the outage, not its peak
Worked through: 500 × 24 ÷ 1000 = 12 kWh
Batteries
Batteries = the larger of kWh ÷ Usable kWh each, and Peak kW ÷ Continuous kW each, rounded up
- Usable kWh each
- From the data sheet
Worked through: 12 ÷ 5 = 2.4, so 3 batteries
The constants, and where they come from
BATTERIES
Published figureUsable energy in kWh and continuous power in kW, by battery
- enphase
- kwh 5, kw 3.84
- powerwall
- kwh 13.5, kw 11.5
- franklin
- kwh 15, kw 10
Applies to: New batteries, at the conditions each data sheet states. Capacity fades over the years
Source: IQ Battery 5P data sheet, Enphase, published 2025-05, read 2026-11.
Also from: aPower 2 data sheet, FranklinWH, published 2026-10, read 2026-11.
Also from: Powerwall 3 datasheet, Tesla, no date on the page, read 2026-11.
Enphase's and FranklinWH's sheets were read directly. Tesla's refused our request, so its row comes from a search excerpt of that sheet, and it should be checked. Models and specifications change.
WATTS_PER_KW
DefinitionalWatts in one kilowatt
Applies to: Definitional, everywhere
True by definition, so there is nothing to cite. The derivation is below.
The prefix kilo means a thousand, so watts times hours divided by 1,000 gives kilowatt hours.
What the labels mean
- Published figure:
- taken directly from the source named beside it.
- Definitional:
- true by definition, like the number of inches in a foot, so there is nothing to cite.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the home battery backup calculator
- 1
List what has to stay on, and estimate its average watts
- 2
Choose how long an outage you want to cover
- 3
Add up what could run at the same time
- 4
Choose a battery, or enter one's specs
- 5
Read the kWh and the batteries, then talk to an installer
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- Average watts over the whole outage. Fridges and pumps cycle, so this is less than their running watts
- Data sheet figures are usable AC energy, so nothing further is divided off
- It checks continuous power, but not the start-up surge of a motor, which needs a check on the data sheet
- It assumes no recharge from solar. A multi-day outage may give you little sun
Pro tips
- Read your electric bill for your real daily kWh
- Ask about a backup panel that keeps the big loads off
- Plan for some loss of capacity over the battery's life