How Long Will a Battery Power a House During an Outage?

By Nacho Iniguez ✦ Updated June 11, 2026

The one equation that answers the whole question

There is no fixed answer to “how long will a battery power my house,” and any number you see without context is close to meaningless. The honest answer comes from one piece of arithmetic:

Runtime (hours) = usable battery capacity (kWh) ÷ your average load (kW)

That is it. A battery holding 13.5 usable kilowatt-hours, powering a house that draws an average of 1 kilowatt, lasts about 13.5 hours. Drop the load to 0.5 kW and you get roughly 27 hours from the same battery. Push it to 3 kW and you are down to about 4.5 hours.

Two variables drive everything: how much energy the battery can actually give you, and how fast your house pulls it back out. Get those two numbers right and the runtime falls out on its own. The rest of this guide is about getting them right.

What “usable” capacity really means

Battery brochures love big total-capacity numbers, but you care about usable capacity, which is almost always a little lower. Most lithium iron phosphate (LFP) home batteries reserve a small buffer to protect cell health, so a battery advertised at 13.5 kWh might give you something very close to that, while older chemistries hold back more.

A few reference points from manufacturer and market specs in 2026:

• A Tesla Powerwall 3 is rated at 13.5 kWh usable, the most common single-unit size on installed systems.
• Portable and DTC units like the EcoFlow Delta Pro 3 sit lower, in the single-digit-to-low-double-digit kWh range per unit, which is why people stack them.
• Stackable installed systems (FranklinWH, Enphase, multiple Powerwalls) let you add capacity in chunks, so “how long” becomes partly a budgeting decision.

If you want to compare real usable numbers across the units worth considering, the best home backup battery roundup for 2026 lays them side by side. For a portable example, the EcoFlow Delta Pro 3 review walks through what one unit actually covers.

Figuring out your hourly load (the part most people skip)

Your average load is where guesses go wrong, because it depends entirely on what you choose to run.

Start with the EIA benchmark: the average US home used about 10,791 kWh per year as of the most recent EIA figures, which works out to roughly 899 kWh a month, or near 30 kWh a day. Spread evenly, that is about 1.25 kW on average. But “average” hides the spikes. Your draw at 3 a.m. with everything asleep is tiny; your draw when the AC compressor and the electric oven kick on together can briefly hit several kilowatts.

During an outage you are not trying to live a normal day. You are trying to keep what matters running. So the practical move is to build your load from the bottom up:

• Refrigerator/freezer: roughly 1 to 2 kWh per day combined, cycling on and off.
• Lights (LED) and phone/laptop charging: a few hundred watt-hours per day total.
• Wi-Fi router, modem, and a TV: modest, often under 0.5 kWh per day combined.
• Well pump or sump pump: small in energy but high in surge, so the battery’s power rating matters here, not just its capacity.

Add those up and an essentials-only profile often lands around 5 to 10 kWh per day. Against a 13.5 kWh battery, that is roughly a day to two days of runtime, which is why a single unit is genuinely useful for most short outages.

Essentials only vs the whole house

This is the fork in the road, and it changes the answer by an order of magnitude.

Essentials backup powers a selected set of circuits: fridge, some lights and outlets, networking, maybe a furnace fan. Your average load stays low, often well under 1 kW, so a single battery can stretch across a full day or longer. This is what most homeowners actually want and what most single-battery installs are designed for.

Whole-home backup keeps everything live, including the energy-hungry loads: central air conditioning, electric resistance heat, an electric water heater, an EV charger. These can each pull 2 to 5 kW or more on their own. Run central air on a hot afternoon and a 13.5 kWh battery can empty in just a few hours. To get meaningful whole-home runtime you typically need multiple batteries and often smart load management that sheds the big draws automatically.

The honest framing: a single battery is an excellent essentials machine and a poor whole-home machine. If you need to cool or heat the entire house through a long outage on batteries alone, you are buying two, three, or more units, and the cost climbs fast. The TOU arbitrage calculator can help justify that capacity if you also plan to cycle it daily for bill savings, not just emergencies.

Why solar resets the clock every morning

Everything above assumes a battery on its own, draining until empty. Pair it with rooftop solar and the math fundamentally changes, because the panels refill the battery during daylight.

In a sunny multi-day outage, a well-sized solar-plus-battery system can settle into a daily rhythm: panels run the house and recharge the battery through the day, the battery carries the house overnight, and the cycle repeats. Instead of asking “how many hours until it dies,” you start asking “can my daily solar production cover my daily outage load?” If yes, your runtime is effectively open-ended for as long as the sun keeps showing up.

The caveats are real. Cloudy stretches cut production. Winter days are short. And not every solar setup keeps producing during a grid outage; the system needs to be designed for islanding, with the battery and inverter configured to run off-grid safely. But when it is built right, solar is the difference between surviving one night and riding out a week. To pressure-test whether the whole investment pencils out, the solar battery ROI calculator and the broader is solar worth it in 2026 guide are the place to run your own numbers.

Putting it together for your house

You do not need an installer’s quote to get a real estimate. You need two numbers and a few minutes.

1. Decide your scenario. Essentials only, or whole home? Be honest about whether you truly need the AC running.
2. Estimate your daily outage load in kWh. Add up the appliances you will actually run. Essentials usually land around 5 to 10 kWh per day.
3. Pick a usable capacity. Compare it against your daily load. Capacity divided by daily load gives you days of runtime; capacity divided by average kW gives you hours.
4. Account for surge. Make sure the battery’s power rating (kW) can handle your largest starting load, like a well pump or AC compressor, not just the total energy.
5. Factor in solar if you have it, since it can extend a one-day battery into a multi-day system.

To skip the back-of-envelope work and get a sized recommendation, run your loads through the battery sizing calculator. It turns the same arithmetic into a concrete capacity target.

The takeaway is steady and unglamorous: a single quality battery will comfortably carry an essentials setup through most outages, often more than a full day. Asking it to run your entire electric home for days, without solar, is asking too much of one box. Size for the scenario you actually face, and the runtime question answers itself. For a full lineup of units matched to those scenarios, start with the 2026 backup battery roundup, or browse all reviews and guides.