How Long Can a Home Battery Power a House?

There is no single answer to this question, and any page that gives you one is hiding an assumption. A home battery holds a fixed amount of energy. How long that lasts depends entirely on how fast you draw it down.

The arithmetic is simple. The honest part is the inputs.

The formula

Runtime in hours equals usable kWh divided by average load in kW.

That is it. Everything else on this page is about getting those two numbers right.

The first number, usable kWh, is not always the number on the box. Batteries reserve a slice at the bottom to protect themselves, which is what depth of discharge measures. Most modern home batteries publish a usable figure at or near their rated figure, but you should still confirm which one a quote is using.

The second number, average load in kW, is the one people guess badly. Your load is not a constant. It rises and falls as appliances cycle on and off, and the average over an outage is what matters, not the peak or the nameplate sum.

If capacity and power are still blurring together, kWh explained sorts them out.

What your loads actually draw

You need rough numbers for the things you plan to keep on. The figures below come from the appliance energy use chart published by Silicon Valley Power, the City of Santa Clara's municipal electric utility, retrieved July 2026. They are estimates based on average operating conditions, and the chart says so. Your own appliances, climate, and habits will move them.

Small, steady loads:

• ENERGY STAR refrigerator, 21 cubic foot top freezer: about 40 kWh per month, roughly 1.3 kWh a day
• CFL/LED bulb (11 watts): about 0.01 kWh per hour
• Laptop: about 0.02 to 0.05 kWh per hour
• Ceiling fan: about 0.025 to 0.075 kWh per hour

Large, intermittent loads:

• Microwave: about 0.12 kWh per 5 minutes, which is roughly 1.4 kW while it is running
• Electric oven: about 2.3 kWh per hour
• Hair dryer: about 1.5 kWh per hour
• Portable space heater rated 1,500 W: about 1.5 kWh per hour
• Window or wall air conditioner, 8,000 Btu: about 0.73 kWh per hour
• Central air conditioner, 3 ton at 12 SEER: about 3.0 kWh per hour
• Electric furnace with fan: about 10.5 kWh per hour

The shape of that list is the whole story. The steady loads are measured in watts. The heating and cooling loads are measured in kilowatts, and one of them can outdraw everything else in your house combined.

Note also what is missing. That chart does not cover well pumps, septic pumps, or EV chargers, and those are exactly the loads that surprise rural and electrified homes. Get real figures for them from the equipment nameplate rather than a general table.

Two scenarios, one battery

Take a battery with 13.5 kWh usable. The Tesla Powerwall 3 publishes exactly that figure on its 2026 datasheet, so it is a convenient anchor.

Essential loads. Refrigerator, ten LED bulbs, a router and modem, phone and laptop charging, a ceiling fan. Add those up from the numbers above and you land somewhere near 0.3 to 0.6 kW on average. At 0.5 kW, 13.5 kWh divided by 0.5 kW is about 27 hours. Stay disciplined and you can stretch past a full day.

Whole-home loads. Add central air conditioning cycling through a hot afternoon, an electric range for dinner, and a water heater. Now your average might sit at 2 to 3 kW, with much higher spikes. At 2.5 kW, that same 13.5 kWh is gone in about 5 hours.

Same hardware. One day versus one evening. The variable is what you choose to keep on, which is why deciding between whole-home and essential backup comes before shopping.

Smaller batteries scale the same way. The Enphase IQ Battery 5P publishes 5 kWh usable, which at 0.5 kW of essential loads is about 10 hours. Larger single units publish more: the FranklinWH aPower 2 at 15 kWh usable, the Fortress Power eVault Max 18.5 at 18.43 kWh usable, the Savant Power Storage 20 at 18 kWh usable (manufacturer datasheets as recorded in our model data).

The other ceiling: continuous power

Capacity tells you how long. The continuous power rating tells you whether a load runs at all. If the things you switch on at the same time exceed the battery's continuous kW, the system will not carry them regardless of how full it is.

Among the models we track, published single-unit continuous output varies widely:

• Enphase IQ Battery 5P: 3.84 kW continuous, 7.68 kW peak
• sonnen sonnenCore+: 4.8 kW continuous
• SolarEdge Home Battery (400V): 5 kW continuous, 7.5 kW peak
• Anker SOLIX X1: 6 kW continuous, 12 kW peak
• LG Energy Solution Home 8: 7.5 kW continuous, 9 kW peak
• Fortress Power eVault Max 18.5: 9.2 kW continuous, 12 kW peak
• FranklinWH aPower 2: 10 kW continuous, 15 kW peak
• Generac PWRcell: 10.5 kW continuous
• Tesla Powerwall 3: 11.5 kW continuous, with no peak figure published
• Savant Power Storage 20: 12.5 kW continuous, 30 kW peak

Those figures come from the manufacturer datasheets recorded in our model data. Two cautions. First, our data does not record how long each peak rating can be held or under what conditions, so treat peak as a short surge allowance and ask for the duration on the datasheet before you count on it. Second, some products publish no peak figure at all, and an unpublished number is not a high number or a low one. It is unknown.

Peak ratings exist because motors are rude. An air conditioner compressor, a well pump, and a refrigerator compressor all draw far more current for a moment at startup than they do while running. That instantaneous surge, not the steady draw, is what trips undersized systems.

What shortens the answer in practice

The formula gives you an optimistic number. Reality trims it.

• Conversion losses. Energy is lost turning stored DC into household AC. Round-trip efficiency is the spec that describes this, and it is worth reading how the number is measured. As one example of why bases matter, Tesla's 89 percent figure for the Powerwall 3 is a solar round trip, measured from solar through the battery to the home, not the AC-to-AC figure that rivals quote. Comparing it directly to an AC-to-AC number is not a fair comparison.
• Motor starts. Every compressor cycle costs more than its steady draw suggests.
• Standby draw. The system itself consumes a little power to stay awake.
• Cold weather. Battery performance and heating loads both move the wrong way at the same time.
• Your own behavior. People open the refrigerator more during an outage, not less.

A sensible habit is to take the arithmetic answer and knock 15 to 25 percent off it before you plan around it.

Solar is what turns hours into days

This is the fork that changes the whole calculation. Without solar, a battery is a tank with one fill. When it is empty, it stays empty until the grid comes back.

With solar that is properly wired and configured to charge the battery during a grid outage, the tank refills every sunny day. The battery carries the night, the array carries the day and tops the battery back up. That combination is the only realistic way to ride out a multi-day outage on storage alone.

Two honest caveats. Not every solar installation can charge a battery while the grid is down; the system has to be configured for islanding, so confirm it explicitly rather than assuming. And output collapses under heavy overcast, so treat sunshine as runtime you might get, not runtime you can count on.

How long an outage do you need to survive?

Runtime math is only useful against a real target. EIA reported that U.S. electricity customers averaged about 11 hours of electricity interruptions in 2024, nearly twice the annual average of the preceding decade, with major events such as Hurricanes Beryl, Helene, and Milton accounting for 80 percent of those hours. Interruptions not caused by major events routinely average about two hours a year. The spread across states is enormous: South Carolina customers averaged nearly 53 hours in 2024, while Arizona, South Dakota, North Dakota, and Massachusetts customers averaged under two (EIA, Today in Energy, December 1, 2025, using Electric Power Annual 2024 data).

That spread is the point. A battery sized for a typical two-hour interruption is a very different purchase from one sized for a hurricane aftermath, and only your own outage history tells you which you are buying. If long outages are your actual worry, read battery vs generator before you decide, because a generator's runtime ceiling is fuel rather than capacity.

The bottom line

Usable kWh divided by average kW gives you hours. Then check the continuous power rating to confirm your loads will run at all, trim the result for conversion losses and real-world behavior, and be honest about whether the battery gets to refill.

For most homes, one battery on essential loads is close to a day, and the same battery on whole-home loads is an evening. Decide which of those you are buying, then run the calculator with your own rate and usage. The full backup picture, including how to choose circuits, is in the home battery backup guide, and the sizing arithmetic in the other direction is in how many kWh you need.