How Long Will a Home Battery Actually Power Your House in an Outage?

"How long will it last?" is the first question most people ask about a home battery — and the honest answer is: it depends entirely on how much you're trying to run. A battery that lasts 24 hours backing up essentials might last only a few hours running your whole house. Here's how to estimate your real number.

The Basic Math#

Runtime comes down to two figures:

• Battery capacity (usable kWh) — how much energy the battery holds. A common single unit stores around 10–13.5 kWh usable. Usable is the word that matters: the chemistry decides how much of the nameplate figure you can actually draw down.
• Your power draw (kW) — how much you're pulling at any moment.

Roughly: runtime (hours) ≈ usable capacity (kWh) ÷ average draw (kW).

So a 13.5 kWh battery running a steady 1 kW load lasts about 13.5 hours. Run a 3 kW load and it's closer to 4.5 hours. The load is the variable you control.

Essentials vs. Whole-Home Backup#

This is the distinction that changes everything.

Backing up essentials only — refrigerator, some lights, phone/laptop charging, internet router, maybe a well pump — a typical single battery can often keep those going for 12–24 hours, sometimes longer, because these loads are modest and intermittent.

Backing up your whole home — including air conditioning, electric heating, an electric water heater, or an oven — draws far more power, and a single battery may only last a few hours. Whole-home backup usually requires multiple batteries stacked together — see whether whole-home backup is worth it.

What Uses the Most Power#

The big energy hogs during an outage are anything that heats, cools, or has a large motor:

• Air conditioning / electric heat — the biggest drains by far
• Electric water heaters, ovens, and clothes dryers
• Well pumps and sump pumps (brief but high surge when starting)

The gentle loads that stretch your runtime:

• Refrigerator/freezer (cycles on and off, modest average draw)
• LED lights, Wi-Fi router, phone/laptop charging

A big part of extending runtime is simply choosing which circuits to back up — many systems let you prioritize essential circuits so the battery isn't drained by an AC unit you don't strictly need during an outage.

Two Things That Change the Picture#

• Solar recharging: If your battery is paired with solar, it can recharge during daylight even while the grid is down (with the right system configuration). This can dramatically extend effective runtime — potentially indefinitely for modest loads on sunny days — turning "hours" into "as long as the sun keeps coming up."
• Continuous power rating: Beyond total capacity, each battery has a maximum continuous output (kW). Some appliances (like AC compressors or well pumps) have high startup surges — a battery with higher continuous power output can start and run these; a lower-output one may not, no matter how much total capacity it has.

How to Estimate Your Own Runtime#

1. Decide what you actually need during an outage (essentials vs. more).
2. Add up the rough power draw of those items. If you don't know your always-on load, splitting a year of bills into heating, cooling and baseload gives you the number that runs around the clock — which is exactly what drains a battery overnight.
3. Divide your battery's usable capacity by that average draw.
4. If you have solar, factor in daytime recharging as a bonus.
5. If the number's too short, the fix is either fewer backed-up circuits or more battery capacity — and the second unit usually costs less per kWh than the first.

If your outages are routinely measured in days rather than hours, capacity stops being the right lever — that is the point at which it is worth comparing a battery against a standby generator.

A good installer will help you size the system to your actual backup goals. The key mindset: a home battery isn't "X hours" in the abstract — it's "X hours of what you choose to run." And a battery bought only for outages sits idle the rest of the year, which is why it's worth knowing whether time-shifting your usage could make it earn something on the other 360 days. This article is general information, not professional advice.