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, and only if the battery's continuous power rating can carry the load in the first place. Checked September 30, 2026.

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.

Usable capacity	Essentials only (~0.5 kW)	Partial home (~2 kW)	Whole home with AC (~5 kW)
5 kWh	~10 hrs	~2.5 hrs	~1 hr
10 kWh	~20 hrs	~5 hrs	~2 hrs
13.5 kWh	~27 hrs	~6.8 hrs	~2.7 hrs
20 kWh	~40 hrs	~10 hrs	~4 hrs
40 kWh	~80 hrs	~20 hrs	~8 hrs

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.

What actually fills the 0.5, 2, and 5 kW columns

The table above hides an assumption: what is actually running at 0.5 kW versus 5 kW. These are the loads that fill those numbers, with running watts, starting or surge watts, and how much of the time each one actually draws power, so the average kW figure is something you can check instead of a number we assert.

Load	Running watts	Starting/surge watts	Typical duty cycle	Rough energy per day
Refrigerator	~500 W	~750 W	Compressor cycles roughly 30-50% of the time	~1.1-1.9 kWh/day (400-700 kWh/year across ENERGY STAR certified models, depending on size and configuration)
Freezer, chest	~500 W	~750 W	Lower than a refrigerator; top-opening design loses less cold air per door open	~0.6 kWh/day (215 kWh/year, ENERGY STAR certified average)
Freezer, upright	~500 W	~750 W	Higher than a chest freezer of the same size	~1.1 kWh/day (395 kWh/year, ENERGY STAR certified average)
Well pump	~150-2,000 W depending on pump size and depth	Often 1,950 W+, higher for larger pumps	Short bursts on demand; commonly 15-30 minutes of total runtime a day for moderate-to-high household water use	Roughly 0.05-1 kWh/day depending on pump size, but size the circuit for the starting watts, not the daily energy
Sump pump (1/3 hp)	~800 W	~1,250 W	Event-driven, not a steady daily cycle; can run continuously for hours in heavy rain or stay off for weeks	No typical daily figure. Size for the starting watts and plan for hours of continuous run during an active storm, not an average day
Furnace or boiler blower and controls	~1,100 W	~2,000 W	Roughly 15-40% of the time on a heating day, more in extreme cold and thermostat-dependent	Roughly 4-10 kWh/day on an active heating day. Electric-resistance heat strips are a separate, much larger load
Ductless mini-split, one 12,000 BTU head	~900 W rated, up to ~1,490 W	Inverter-driven, so it modulates compressor speed instead of cycling fully on and off, meaning it can hold close to its rated draw continuously in extreme heat or cold rather than following a simple duty cycle	Roughly 5-15 kWh/day depending on outdoor temperature and hours run
Central air conditioner (3 ton)	~5,400 W	~7,200 W	Roughly 40-65% on a typical hot afternoon, higher on extreme-heat design days	Roughly 50-85 kWh/day on a hot day. We could not find a dated nameplate spec specific to a 3.5-ton unit this run; a larger system draws more on every column here
A few key lights	~200-600 W combined	Negligible	On when needed, not a duty cycle	Varies with hours used
Wifi router, phone and laptop charging	Under 100 W	Negligible	Continuous	~1-2 kWh/day

Running and starting watts are this site's own already-published circuit reference, reused from the whole-home versus essential-loads guide and re-checked live this run. Duty cycle and daily-energy figures are a planning range built from the sourced running watts plus published duty-cycle guidance, not a measurement of your specific home. Full sourcing is in the "What we could not verify this run" section below.

The practical use of this table: your average-kW figure is not one number all day. A refrigerator alone drawing 500 W at a 40% duty cycle averages about 200 W, not 500 W, but a well pump's starting surge can still trip a battery even though its daily energy use is small. Peak power and daily energy are different questions, and a quote that only discusses one of them is incomplete.

What a battery can actually start: continuous and surge power by system

Energy capacity does not decide whether a load starts. A battery's continuous power rating, and its short-duration surge rating, decide whether the well pump or the central AC compressor actually turns on.

System	Continuous power (per unit)	Surge/peak power	Source
Tesla Powerwall 3	11.5 kW AC continuous	185 A LRA motor-start current; Tesla does not publish a duration for that figure	Tesla's own Powerwall 3 datasheet
Enphase IQ Battery 5P (US)	3.84 kW continuous	7.68 kW for 3 seconds, 6.14 kVA sustained to 10 seconds	Enphase's own IQ Battery 5P datasheet, dated May 2025
FranklinWH aPower 2	10 kW continuous	15 kW peak for 10 seconds, 25 kW transient for 1 second	FranklinWH's own aPower 2 datasheet

These figures are reused from this site's already-verified benchmarks in the best home batteries comparison and how to read a battery quote, checked against manufacturer datasheets earlier this month. Set against the appliance table above: a single Enphase IQ Battery 5P's 3.84 kW continuous rating is already close to the running watts of a central AC plus a furnace blower and a refrigerator combined, before any starting surge, which is why an Enphase whole-home design with AC commonly needs more than one unit. Tesla Powerwall 3 and FranklinWH aPower 2 both publish enough continuous power for that same combination on a single unit.

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. See the whole-home versus essential-loads decision for the battery-count math.
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

List the loads you truly need during an outage, not the whole house. The essential loads panel guide covers what usually belongs on that list.
Add up their running watts to get an average kW figure, checking your list against the appliance table above instead of guessing.
Divide your usable kWh by that number for a rough hours estimate. The kWh sizing guide walks through this in more depth.
Check that the battery's continuous kW rating, and its surge rating, can actually carry your peak loads at once.
If outages in your area run longer than a day, plan for solar recharge or a second battery rather than a bigger single unit.

What we could not verify this run

No single dated primary source publishes a per-appliance duty cycle percentage for a general household. Duty cycle depends heavily on climate, insulation, equipment age, sizing, and how a household actually uses the appliance. The ranges above are cross-checked across multiple independent technical and consumer-energy publishers for order of magnitude, not a lab measurement of your home, and we labeled the resulting daily-energy figures as a planning range rather than a precise number.
The freezer annual-kWh figures (215 kWh chest, 395 kWh upright) are fetched directly from ENERGY STAR's own products page. The refrigerator range (400-700 kWh/year) comes from a third-party analysis of ENERGY STAR's certified refrigerator list, since EPA's own overview page does not publish a single average figure; we present it as a range rather than the single point estimate that analysis reported.
We could not find a dated, manufacturer-published nameplate spec for a 3.5-ton central air conditioner this run, so the central AC row reuses the same 3-ton reference this site already cites in the whole-home versus essential-loads guide. A 3.5-ton unit draws somewhat more on every column here; get your installer's actual nameplate rating rather than scaling this table yourself.
Sump pump and mini-split loads are not modeled as a steady daily kWh figure because they are event-driven or inverter-modulated rather than duty-cycled the way a refrigerator or central AC compressor is. Size those circuits for their surge or rated continuous watts, not an average-day energy number.
Tesla's and Enphase's own spec pages again returned a blocked automated fetch this run (Tesla HTTP 403, Enphase page moved), consistent with prior runs logged elsewhere on this site. Those two systems' power figures are carried forward from this site's own already-verified numbers rather than re-fetched live this run.

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.

Why does a well pump or sump pump trip a battery that has plenty of energy left?

Because energy and power are different limits. A well pump can draw close to 2,000 starting watts or more for a second or two depending on its size and depth, which can exceed a battery's surge rating even when the battery has most of its usable kWh still stored. Check the starting watts against the battery's surge rating, not just its continuous rating.

Sources

Department of Energy: Homeowner's Guide to Going Solar
ENERGY STAR: Freezers (chest and upright annual kWh figures, fetched live September 30, 2026)
Howell-Oregon Electric Cooperative: Choosing a Generator, Which Size Is Right? (running and starting watts by circuit, re-checked live September 30, 2026)
Tesla Powerwall specifications
Enphase IQ Battery 5P Data Sheet
FranklinWH aPower 2 Datasheet