How long can a battery run my home?
There is no single number. A rough estimate is the battery's usable kilowatt-hours (kWh) divided by the load it is powering in kilowatts (kW). In Florida, a big factor is whether air conditioning runs on the battery, and the real answer for any home depends on system design.
Two numbers to know: kWh and kW
A battery has two ratings. Energy capacity, in kilowatt-hours (kWh), is how much it can store. Power capacity, in kilowatts (kW), is how much it can deliver at any one moment (U.S. Department of Energy). A kWh is one kilowatt over one hour (U.S. Energy Information Administration), so hours of backup are roughly the stored kWh divided by the kW your home is drawing.
In a national lab's reference data, the representative home battery is 5 kW / 12.5 kWh, which it calls a 2.5-hour system: 12.5 ÷ 5 = 2.5 hours at full power (National Laboratory of the Rockies, formerly NREL).
The kW limit matters too. Air conditioning and electric heat can draw a lot of power for short moments, which may be more than a system can supply at once (Lawrence Berkeley National Laboratory). Some energy is also lost going into and out of storage (U.S. Department of Energy); the national lab's reference data assumes about 85% comes back out (National Laboratory of the Rockies).
What the battery is asked to power
Backup can cover a short list of essentials or the whole house, and a 2022 national study by Berkeley Lab with NREL modeled both (study page). Its results depended first on system size and on which loads were chosen. With solar recharging the battery, essentials such as the fridge, night-time lights and internet were easier to keep running than a list that also included heating and cooling. What a given system can cover depends on its design.
In Florida, air conditioning is the big one
In 2020, 96% of Florida households used air conditioning, most of them with central units (EIA). Florida homes with AC used about 4,400 kWh a year on it: close to twice the U.S. average, and more than any other use EIA lists separately, such as water heating or refrigerators (EIA table CE4.6).
The national study found backup tended to cover less where cooling loads are large, including parts of the Southeast. In Harris County, Texas, which it examined closely, AC efficiency and thermostat settings each made a clear difference in summer (Berkeley Lab).
Can solar recharge it during the day?
Only if the system is set up so the panels keep working while the grid is down. Most home solar systems need the grid connection to supply power; a battery or generator is what can bring them back online in an outage (U.S. Department of Energy). Whether a given system can recharge in an outage depends on system design.
Where solar can recharge, the Berkeley Lab modeling (which used typical weather) found longer outages reduced backup only modestly because the panels refilled the battery each day (Berkeley Lab).
Weather
Recharging needs sun. When the lab replayed ten real outages, the hurricanes came out worst, partly because clouds cut solar output. In its model of Hurricane Florence, the panels produced almost nothing for the first three days of a roughly eight-day outage; Hurricane Irma, with one day of reduced sun, came out better (Berkeley Lab). The study also assumed the equipment came through undamaged, which a real storm may not allow.
Settings and starting charge
Home batteries are usually set to hold some charge back in case the power goes out. A higher reserve leaves more for an outage and less for day-to-day use. Owners can often adjust it in an app, and some manufacturers offer a feature that raises it ahead of a storm (Berkeley Lab, 2024). What a given battery offers depends on the product and system design.
How full the battery is when the power goes out also counts. The 2022 study's long-outage modeling assumed people could charge their batteries before the event (Berkeley Lab, 2024).
A simple way to estimate
Hours of backup ≈ usable battery kWh ÷ average load in kW
A hypothetical example. These round numbers are not typical for any home or product:
- A battery with 10 kWh of usable energy.
- Loads averaging 0.5 kW: 10 ÷ 0.5 = about 20 hours.
- Loads averaging 3 kW: 10 ÷ 3 = a little over 3 hours.
The formula leaves out solar, the kW limit, losses and the reserve, so it is a starting point only. Real results depend on system design.
For your own numbers, divide the kWh on your electric bill by the days in the billing period, then by 24, for an average kW. The loads you back up would be some share of that, and an average hides the peaks. Use a bill from the season you are planning for.
Questions to ask
- How many usable kWh, and how many kW?
- Which circuits would be backed up, and is the AC one of them?
- Can the panels recharge the battery during an outage?
- What reserve setting is planned, and can it be raised before a storm?
A consultant from a Florida-licensed solar contractor can go through these for your home.
General information only, not electrical, engineering or financial advice.
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