Home battery backup: how it works and what it powers

What home battery backup does

A home battery holds electricity until you need it. When the grid goes down, it can feed your home instead. One maker, FranklinWH, puts it simply: "With a home battery, homeowners can store energy and use it when the grid is down." ³

That's the whole idea. The battery isn't a generator. It burns no fuel, so there's no exhaust and no refueling. It also won't last forever, because it only holds what it stored.

Terms you'll see on every product page:

- kWh (kilowatt-hour) is how much energy the battery stores. Think of it as the size of the tank. - kW (kilowatt) is how much power it can deliver at one moment. Think of it as how wide the tap is. - Whole-home backup means the battery can run the whole house. Partial backup means it runs only some circuits.

Whether one battery covers your whole house depends on your loads and the battery you pick.

What a home battery can power

It depends on the size of the battery and what you ask of it. FranklinWH says: "A small battery may be sufficient for homeowners who only want to power a few appliances or devices during a power outage." And: "A larger battery may be necessary for homeowners who want to power their entire home during an outage." ³

Two numbers decide it. The first is the power rating, which limits what can run at once. The second is stored energy, which limits for how long.

Here are real specifications from two makers. Both come from the maker's own pages, checked on 2 November 2026.

- Tesla Powerwall 3: nominal battery energy of 13.5 kWh, and nominal output power options of 5.8, 7.6, 10 or 11.5 kW. Tesla publishes these figures. ¹ - Enphase IQ Battery 5P: "a total usable energy capacity of 5.0 kWh" and "3.84 kW of continuous power." ²

These aren't directly comparable as a pair. Tesla's footnotes tie its figures to stated conditions. ¹ Enphase says its battery can provide backup "when installed with IQ System Controller 3/3G." ² Read each maker's datasheet for the conditions behind a number.

Several units can be combined. FranklinWH notes that "home batteries are modular, which means that multiple batteries can be connected to create a larger storage capacity." ³

How long it will run: a worked estimate

You can estimate runtime with one division: stored energy in kWh divided by the load in kW. The result is hours.

Suppose you want to run a refrigerator, a few lights, a router and phone chargers. Say that adds up to 0.5 kW. This is an example, not a measurement. Check the labels on your own appliances.

- Enphase IQ Battery 5P: 5.0 kWh ÷ 0.5 kW = 10 hours. ² - Powerwall 3: 13.5 kWh ÷ 0.5 kW = 27 hours. ¹

Both are rough estimates. They ignore energy lost in conversion and any reserve the system keeps, and the makers' pages we read don't give a runtime for this load. A heavier load drains the battery faster. Run a 3 kW load, and the same 5.0 kWh lasts about 1.7 hours (5.0 ÷ 3). That 3 kW load is within the 3.84 kW rating, but only just. ²

Solar panels can recharge a battery during a long outage. That needs its own explanation, so we leave it out here.

What installation involves

A battery that powers your house is wired into your electrical system. That makes it a job for a licensed electrician, with a permit where your local rules require one. Don't treat it as a weekend project, and don't look for wiring steps here. We don't give any.

The reason is isolation. When your home runs on its own power, it must be kept apart from the utility lines. If it isn't, power can flow back out onto them. This is called backfeeding. The Consumer Product Safety Commission (CPSC) describes it as "a dangerous practice known as backfeeding that that can result in serious injury or death from electrocution or fire." ⁴

Utility crews face the same risk. OSHA tells line workers that "lines may become energized as a result of backfeed from portable generator use." ⁵

Those two pages are about generator use, not batteries. We use them to show why isolation matters. For a battery, follow the maker's installation manual and your installer's advice.

Questions to ask an installer:

- Which circuits will the battery back up, and which won't it? - What's the continuous output, and what happens if I start a large motor? - Is a permit needed, and who handles it? - What does the maker's warranty cover? Tesla lists a warranty of "10 years" for Powerwall 3, and Enphase describes a "15-year limited warranty up to 6,000 cycles" for the IQ Battery 5P. ^{1 2}

What it costs

We can't give you a reliable price. Neither maker's page we read states one. Enphase's page says: "To purchase this product, please contact your local authorized Enphase distributor." ²

The battery is only part of the bill. Installation, the electrician's work and any extra equipment are quoted separately by installers. Get written quotes that list each item.

Tax credits and rebates can change the final cost. They change often, so check the official source for your area and talk to your tax adviser. We won't tell you what you'll qualify for or save.

Don't build or modify a battery

Buy a complete product from a maker and have a licensed electrician install it. Don't build a pack from loose cells and don't modify one. This guide doesn't explain how, and neither should any guide aimed at homeowners. Ask the maker or installer if you have questions about a product.