A blackout is the moment a home battery has to earn its price, and the first question owners ask is simple: how many hours will it last? The honest answer depends far more on what you keep running, and on how full the battery is when the grid fails, than on the battery itself. Here is the math, using Tesla’s own specifications and U.S. government data on how much electricity homes use.
Quick answer
A Powerwall 3 is rated at 13.5 kWh[src]. Starting full, and stopping when Tesla’s 10%[src] standby threshold is reached, it would run a home using the U.S. average of 28.4 kWh/day[src] for about 10.3 hours. Cut your load to essentials averaging 0.5 kW and the same battery lasts about 24 hours. With solar panels recharging it, it can keep going for days.
Key takeaways
- Runtime = usable energy in the battery ÷ the average power your home draws during the outage.
- At the U.S. average use, one Powerwall 3 lasts about 10.3 hours from full; trimming to essentials can more than double that.
- How full the battery is when the outage starts matters as much as its size. Your Backup Reserve setting decides that.
- Air conditioning is the largest single use of electricity in U.S. homes and the fastest way to drain a battery.
- With solar, Powerwall can recharge during the day and stretch a multi-day outage.
For the bigger picture on sizing and costs, start with our home battery guides.
How do you calculate how long a Powerwall will last?
Two numbers decide runtime: energy (kWh), which is how much the battery stores, and power (kW), which is how fast your home uses it. Divide the first by the second:
Runtime (hours) = usable battery energy (kWh) ÷ average load during the outage (kW)
Tesla’s datasheet rates Powerwall 3 at 13.5 kWh of AC energy at 25 °C (77 °F) when new. Tesla’s support pages add an important detail: when less than 10% of its energy remains, Powerwall goes into standby and stops powering the house. That leaves about 12.15 kWh to use from a full charge, which is the figure we use in every calculation on this page.
| Average load during the outage | 1 Powerwall 3 | 2 Powerwall 3 units |
|---|---|---|
| 0.5 kW | 24.3 h | 48.6 h |
| 1 kW | 12.2 h | 24.3 h |
| 1.5 kW | 8.1 h | 16.2 h |
| 2 kW | 6.1 h | 12.2 h |
| 3 kW | 4.1 h | 8.1 h |
| 5 kW | 2.4 h | 4.9 h |
The key word is average. A fridge compressor, a well pump or a microwave draws far more than the average for a few minutes, and that’s fine: what drains the battery is the total energy used over the hours of the outage.
How long would it run an average home in your state?
The U.S. Energy Information Administration (EIA) publishes how much electricity the average residential customer buys each month. In 2024 the national average was 863 kWh/month[src]. Spread evenly over the day, that is an average load of about 1.18 kW, but the averages vary a lot by state:
| State | Average use per home | Average load | 1 Powerwall 3 | 2 units |
|---|---|---|---|---|
| Texas | 1,096 kWh/month | 1.5 kW | 8.1 h | 16.2 h |
| Florida | 1,104 kWh/month | 1.51 kW | 8 h | 16.1 h |
| Arizona | 1,075 kWh/month | 1.47 kW | 8.3 h | 16.5 h |
| California | 503 kWh/month | 0.69 kW | 17.6 h | 35.3 h |
These are whole-home estimates with everything running as usual. Two things make the real number different for your house:
- Season. Annual averages blend mild months with hot ones. In a Texas or Florida summer, daily use and therefore the drain on the battery is higher than the table suggests.
- Your own habits. Your utility bill or online account shows your actual kWh per month or per day. Divide daily kWh by 24 for your average load, then use the table above, or enter it in our battery size calculator.
Why the battery’s charge when the outage starts matters
Most Powerwalls spend normal days in Self-Powered or savings modes, using stored energy to cut your bill. Only the part you set aside as Backup Reserve is guaranteed to be waiting when the grid fails. In the Tesla app you can choose any reserve from 0% to 80%[src].
That setting changes everything. If an outage starts while the battery is sitting at a 20% reserve, the energy between 20% and the 10% standby point is only about 1.35 kWh, enough for roughly 1.1 hours at the U.S. average load. At an 80% reserve you would have about 9.5 kWh, or around 8 hours.
Tesla also notes that during an outage the battery may discharge below your reserve percentage, so the reserve is a floor for everyday use, not a hard limit on backup.
Can a Powerwall run central air conditioning?
Power-wise, often yes. Powerwall 3 delivers up to 11.5 kW[src] of continuous power and, per Tesla’s datasheet, can start heavy loads rated up to 185 LRA (locked-rotor amps), which Tesla says means a single unit can support the power needs of most homes. Compare that with the LRA printed on your AC unit’s nameplate and confirm with your installer.
Energy-wise, AC is the problem. According to the EIA, air conditioning accounts for about 19%[src] of the electricity used in U.S. homes, more than any other single use. That is why Tesla’s own advice for extending backup is to cut energy-intensive loads such as air conditioners, vehicle charging, electric heaters and dishwashers.
Running AC on battery during an outage
Pros
- Powerwall 3 has the continuous power for many central AC systems
- The 185 LRA load-start rating is designed for heavy motor loads
- Cooling during a heat-wave outage can be more than a comfort issue
Cons
- AC is the largest electricity use in U.S. homes and drains the battery fastest
- Cooling a whole house for hours eats into a single unit quickly
- Large or older systems may still exceed the start-up rating
With solar vs without solar: multi-day outages
Without solar, a Powerwall is a fixed tank: once it reaches the 10% standby point, the house goes dark until the grid returns.
With solar, it can refill. Tesla says Powerwall can recharge from your panels to keep running the home while the grid is down, and that if it does drop into standby, it automatically tries to recharge from solar every hour between 8 AM and 4 PM. Powerwall 3 accepts up to 5 kW of continuous charging per unit, according to its datasheet. A traditional grid-tied solar system without a battery does not work during a blackout.
The catch is weather. Tesla warns that storms can mean outages lasting “days, not hours,” with lower solar production, and that snow or debris on panels reduces output. On cloudy days, treat solar as a bonus and the battery’s stored energy as what you can count on. Our solar savings calculator helps you size the panels that would feed it.
Is one Powerwall enough for your home?
It depends on whether you want to back up essentials or the whole house, and on how long outages last where you live.
| Spec | 1 unit | 2 units |
|---|---|---|
| Usable energy before standby | 12.15 kWh | 24.3 kWh |
| Continuous power | 11.5 kW | 23 kW |
| Runtime at the U.S. average load | 10.3 h | 20.5 h |
| Runtime at 0.5 kW (essentials) | 24.3 h | 48.6 h |
At the national average load, one unit covers an evening-length outage, and much longer if you limit use to the fridge, lights, internet and a few outlets. Whole-home backup through a summer outage, or a home with electric heat, is where a second unit or solar recharging starts to matter.
How to estimate your own runtime in three steps
- Find your daily use. Check your utility bill or app for kWh per day (or kWh per month ÷ 30). For outage planning, use a bill from the season you’re worried about.
- Decide what you’ll run. Whole home, or essentials only? Many utility apps show hourly use, which is the best guide to your real average load.
- Do the division. Usable energy (about 12.15 kWh per Powerwall 3, from full) ÷ your average load in kW = hours. Or let the battery size calculator do it, including reserve and efficiency.
Get quotes for a backup battery
Installer quotes vary widely for the same equipment. Compare at least three before you sign, and ask each installer what backup reserve, which circuits and what runtime they are designing for.
