It's the first question almost everyone asks, and the honest answer is that it depends on two numbers: how much energy the battery holds, and how fast your house uses it. Once you know both, the math is simple.
Two numbers on every battery
A home battery is rated two ways. Its capacity, in kilowatt-hours (kWh), is how much energy it stores, like the size of a tank. Its power rating, in kilowatts (kW), is how much it can deliver at once, more like the width of the hose.
Capacity decides how long the battery lasts. Power decides what you can run at the same time. A battery with plenty of stored energy but too little power output can't start a big load like central AC, even when it's full.
The basic math
Divide the battery's capacity by what the house is drawing. A battery holding 10 kWh that's running 1 kW of fridge, lights, fans and Wi-Fi lasts about 10 hours. Run 3 kW and it lasts about 3 hours. In real life you get a bit less than the label, because some energy is lost converting it for the house.
Loads also change through the day. The fridge cycles on and off, and lights only matter after dark. So a battery often lasts longer than a straight division suggests, as long as the big loads stay off.
Your electric bill already has the answer
Your bill lists how many kWh the house used that month. Divide by the number of days in the billing period to get a daily average. A home that used 1,200 kWh in a 30-day month averages 40 kWh a day.
That number tells you a lot. It's what your house uses with everything running normally, AC included. Backing up all of it takes a much bigger system than backing up the essentials. That's why we size systems from the bill instead of guessing.
Why AC changes everything in Florida
In most Gulf Coast homes, central AC is the biggest single load by a wide margin, and from May through October it runs most of the day. A battery that keeps a fridge, lights and Wi-Fi going for a day or more can drain in a few hours if it also has to carry a large AC unit.
So the first decision is whether you want AC covered during an outage. Some homeowners do, especially with older family members or medical needs in the house. Others are fine with fans and keeping the fridge cold. Both are reasonable. They lead to different system sizes.
What a battery usually covers first
When people pick what matters during an outage, the list tends to look like this:
- the fridge and freezer
- lights in the rooms you use
- Wi-Fi and phone charging
- ceiling fans
- medical equipment like CPAP machines or oxygen concentrators
- a few kitchen outlets
Those draw far less than AC, which is why a battery sized for them can last a long time.
Solar stretches it out
On its own, a battery runs until it's empty and then waits for the grid. With solar panels, it can refill every day the sun is out, even while the grid is down. During a long outage, that daily recharge is what keeps a house running for days instead of hours.
Getting your own number
The quickest way to know how long a battery would run your house is to look at your actual use. During a free in-home assessment we look at your electrical panel, ask what you want running when the power is out, and size the system from your electric bill. You can read more about whole-home battery backup first if you like.