How Long Will a Home Battery Last? Backup Runtime by Size

A home battery's backup runtime is roughly its usable capacity in kWh divided by your average load in kW. Powering only essentials can last a day or more, while running the whole home with air conditioning may last just a few hours on the same battery.

Runtime is not one number. The same battery can last a full day or barely an evening depending on what you keep running. So the useful question is not "how long does a battery last" but "how long does it last for the loads I actually need during an outage".

Backup hours by usable capacity

These estimates divide usable capacity by a steady load. They are a planning starting point, not a guarantee.

Essentials means a fridge, some lights, wifi, and phone charging. Partial home adds more circuits, a well pump, or a modest window unit. Whole home with AC means central air conditioning running alongside everything else.

The assumptions behind the numbers

• Usable, not nominal. Manufacturers advertise a nominal or nameplate capacity, but backup runs on usable capacity, which is what you can actually draw after the reserve and depth-of-discharge limits. Some popular systems are rated around 13.5 kWh usable. Size backup from the usable figure, not the headline number.
• Continuous-power cap. A single home battery commonly supplies somewhere between about 3.8 kW and 11.5 kW continuous depending on the model, with a brief higher surge. The whole-home-with-AC column sits near that ceiling, so running central AC plus the rest of the house often needs more than one battery, no matter how much energy is stored.
• Startup surge. Motors in an air conditioner, well pump, or refrigerator draw a short surge well above their running watts. A battery can trip or refuse to start those loads even when average demand looks fine on paper.
• Real-world losses. Inverter and round-trip losses of roughly 5 to 10 percent, plus cold temperatures, shorten actual runtime below the simple math above.

How to size for your outage

1. List the loads you truly need during an outage, not the whole house.
2. Add up their running watts to get an average kW figure.
3. Divide your usable kWh by that number for a rough hours estimate.
4. Check that the battery's continuous kW rating can actually carry your peak loads at once.
5. If outages in your area run longer than a day, plan for solar recharge or a second battery rather than a bigger single unit.

Commercial note

Home Battery Guide may earn from affiliate links or flat-fee referrals to named vetted installers. Rankings do not move with compensation. We do not sell the same lead to multiple installers, and a referred installer quote still has to pass the same quote check.

Frequently Asked Questions

How long will a 13.5 kWh battery run my house?

For essentials only at roughly 0.5 kW, about a day. Running a partial home near 2 kW, closer to seven hours. Running central AC and the whole house near 5 kW, only a few hours, and that is also near a single battery's power limit.

Does solar make the battery last longer during an outage?

Yes, if your system is set up for it. Solar can recharge the battery by day, which extends a multi-day outage well beyond the stored kWh, though output drops in clouds and at night.

Why does my battery not last as long as the spec sheet suggests?

Spec sheets quote ideal conditions. Real runtime is shorter because of inverter losses, cold weather, startup surges from motors, and because you are usually running more than the minimum load.

Should I buy a bigger battery or a second one?

A second battery adds both stored energy and continuous power, which helps if you want to run large loads like AC. A single larger unit adds energy but is still capped by its own power rating.

Sources

• Department of Energy: Homeowner's Guide to Going Solar
• Tesla Powerwall specifications
• Enphase IQ Battery home storage