How long will your battery run the house in a blackout?
By Sithum Imarsha Last reviewed
Backup time = kWh left in the battery ÷ the average watts you keep running. A full 13.5 kWh Powerwall 3 on a fridge, Wi-Fi, lights and a TV (2.15 kWh a day) lasts about 6.3 days.
Your result
Backup time
6.3 days
13.5 kWh ÷ 2.15 kWh a day = 6.3 days
- In hours
- 151h
- Average draw
- 89W
- Running at once
- 315W
- Backup output
- 5kW
Next load? Add a freezer (46 W average) and it drops to 4.2 days.
Buying a battery? Work out the federal rebate.
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How we calculated this
Energy when the power goes out
kWh available = usable kWh × starting charge. Powerwall 3's 13.5 kWh is already AC, after its inverter, so we take nothing more off.
13.5 kWh × 100% = 13.5 kWh
Energy a day
kWh a day = the sum of watts × hours a day ÷ 1,000. A fridge's compressor switches on and off, so we use its average watts over the day, from a yearly kWh figure.
2.15 kWh a day, an average of 89 W
Backup time
Hours = kWh available × 1,000 ÷ average watts.
151 hours, 6.3 days
Can it run them at once?
Energy is not the only limit. The battery can only supply its backup output at one time. We add the running watts of every load you run and compare. For a fridge or freezer that is its compressor's running watts (150 W, Estimate), not its daily average. A motor (an air conditioner or pump) starts at a higher current than it runs: compare its locked rotor amps (LRA) on the nameplate with the battery's.
315 W against 5 kW
| Load | kWh a day |
|---|---|
| Fridge | 1.1 |
| Freezer | 0 |
| Lights | 0.25 |
| Wi-Fi modem | 0.36 |
| TV | 0.4 |
| Phone and laptop charging | 0.04 |
| Load 1 | 0 |
| Load 2 | 0 |
| A day | 2.15 |
| Load | Watts | Hours a day | Where it comes from |
|---|---|---|---|
| Fridge | 46 W (Estimate) | 24 h (Estimate) | Average watts from the appliance preset: 400 kWh a year, a 2 to 3 star family fridge |
| Freezer | 46 W (Estimate) | 24 h (Estimate) | Same as the fridge preset: 400 kWh a year. Use your label |
| Lights | 50 W (Estimate) | 5 h (Estimate) | Our pick: 5 LED bulbs × 10 W (Champion Power Equipment generator wattage chart (US, 120 V): 8–15 W a bulb) |
| Wi-Fi modem | 15 W (Estimate) | 24 h (Estimate) | Our pick in the middle of 10–20 W (Champion Power Equipment generator wattage chart (US, 120 V)) |
| TV | 80 W (Estimate) | 5 h (Estimate) | Appliance running cost preset: Our pick near the low end of two ranges for a 55 inch TV: Home Upkeep 70 to 110 W, EcoFlow AU 70 to 130 W (LED or LCD) |
| Phone and laptop charging | 20 W (Estimate) | 2 h (Estimate) | Our pick at the low end of 20–100 W while charging (Champion Power Equipment generator wattage chart (US, 120 V)) |
| Figure | Value | Status |
|---|---|---|
| Usable energy | 13.5 kWh AC, 25 °C, when new | Official source |
| Continuous output | 5, 10, 11.04 kW by model | Official source |
| Motor start | 185 A locked rotor amps | Official source |
| Maximum charge | 5 kW | Official source |
| Round trip | 89% solar to battery to home | Official source |
- Ask your installer whether your system is set up for backup, which circuits it covers, and whether solar recharges it during an outage. Tesla's datasheet lists the Backup Gateway 2 as Powerwall 3's islanding device.
- We start from the lowest Powerwall 3 output, 5 kW. Type yours if it is set up for 10 or 11.04 kW.
- We do not add any solar recharge during the outage. We do not count the battery's own standby use either: we found no published figure.
- Only Powerwall 3 is a preset because we read its full datasheet. For another battery, type its usable kWh and backup kW from its datasheet.
- For a medical device, use the figure on its own power supply. This is not medical advice. If you rely on one, ask your supplier about backup power.
“Official source” means read in full on the maker’s own datasheet, manual or spec page, or in an official dataset (BOM). “Dated third-party source” means read on a dated third-party page, such as a product test. “Not yet checked” means one non-official source. “Estimate” means our own pick or reasoning: type your own figure.
Sources
Backup time is set by your loads
A battery does not run “a house”. It runs whatever is switched on. Backup time is the energy left in the battery divided by the power your loads draw.
- Energy left = usable kWh × the charge when the power goes out. A full Powerwall 3 has 13.5 kWh.
- Power drawn = the average watts of everything you keep on. A fridge, Wi-Fi, lights for the evening and a TV average about 90 W.
13.5 kWh ÷ 90 W is about 150 hours: six days. Add a 2,000 W heater for four hours a day and that drops to under two days.
Two limits, not one
- Energy (kWh) sets how long.
- Output (kW) sets how much at once. Powerwall 3 supplies 5, 10 or 11.04 kW continuously, depending on how it is configured. An oven, a kettle and a heater together can go over 5 kW.
Motors are a third check. An air conditioner or pump draws a large current for a moment as it starts. Tesla rates Powerwall 3 at 185 A locked rotor amps (LRA). Compare that with the LRA on the motor’s nameplate.
Check your install before you need it
Ask your installer whether your system is set up for backup, and whether solar recharges it during an outage. Tesla’s datasheet lists the Backup Gateway 2 as Powerwall 3’s islanding device. Questions to ask:
- Is my battery set up for backup?
- Does my solar recharge the battery during an outage?
- Which circuits are backed up?
- What backup output (kW) is it set to?
- What backup reserve is set in the app?
Getting more hours
- Keep a backup reserve. A higher reserve in the app means more energy when the grid fails, and less for daily savings.
- Switch off what you do not need. A second fridge or a freezer running all day is about 1.1 kWh a day each at 400 kWh a year.
- Cook and heat with the stove, kettle and heater in short bursts, not all at once.
If you are still choosing a battery, the battery rebate calculator shows the federal discount on its usable kWh.
Questions
How long will a 13.5 kWh battery run my house in a blackout?
It depends on what you run, not the house. On a fridge, Wi-Fi, lights and a TV (about 2.15 kWh a day) a full Powerwall 3 lasts about 6.3 days. Run an oven or a heater and it is hours, not days.
How long will a 10 kWh battery last in a blackout?
About 4.7 days from full on the same essentials (about 2.15 kWh a day), if all 10 kWh is usable AC energy. Pick "Another battery" and type your own usable kWh and loads.
Will my battery work in a blackout at all?
Ask your installer whether your system is set up for backup, and which circuits it covers. Tesla’s datasheet lists the Backup Gateway 2 as Powerwall 3’s islanding device.
Why does the backup output matter as well as the kWh?
The kWh say how long. The backup output (kW) says how much can run at once. Powerwall 3 gives 5, 10 or 11.04 kW continuous, depending on the configured model. Go over it and the battery can trip.
What starting charge should I use?
If you set a backup reserve in the app, use that: the battery keeps that share for an outage. For the worst case, use the lowest your battery reaches on a normal evening.
Does solar recharge the battery during a blackout?
Ask your installer whether solar recharges your battery during an outage. We do not count any solar here, so if yours does, the battery can last longer than shown.
Where do the fridge and TV figures come from?
The fridge uses 400 kWh a year, a 2 to 3 star family fridge (our pick; YourHome gives 232 to 427 kWh for a 440 L fridge). The TV is a 55 inch LED at about 80 W. Type your own figures for a better answer.