How long would a battery run your house?

Solar that makes more than you use changes the question

Most battery calculators treat the battery as a tank that empties. That is right for a battery on its own. It is the wrong picture for a battery charged by solar panels sized to make more than your home uses in a year, which proposals call an offset over 100%.

With a system like that, an outage on an ordinary day goes like this. Through the sunny part of the day the panels make more than the house draws, so they run the house and put the extra into the battery. After dark the battery runs the house. At sunrise the panels take over again and the battery refills. The battery never has to last the whole outage, only the dark hours of each night, so on an average day you can keep using power the way you normally do with the grid down.

Three things decide whether that holds for a given home: whether the battery holds enough for your night-time use, whether the battery's power rating covers what runs at once (the air conditioner starting is the big one), and whether the day is sunny enough.

A dark, rainy day can make a fraction of the usual output, and that is when the battery-alone numbers apply. Choose "Sized to my yearly use" below and enter the offset from your proposal to see both.

Can I keep using my normal power in an outage with solar and a battery?

On an average day, yes, if the panels make more than your home uses and the battery can carry the night. In daylight the panels run the house and refill the battery; after dark the battery runs the house until the sun comes back. That is the point of sizing solar above your usage. It needs two things from the equipment: a system set up to keep the panels running when the grid is down, and a battery with enough power rating for what runs at the same moment.

How long will a battery run my house without solar?

Divide the battery's usable kWh by what your home uses per hour. Your bill gives you the second number: a month's kWh divided by the hours in that month. A home that used 1,500 kWh in a 31-day month averaged about 2 kW, so a battery with 13.5 usable kWh (an example size, not a plan size) lasts roughly 7 hours at normal use and about twice that if you run half as much. The same numbers apply to a solar home on a dark, rainy day, when the panels make little.

Why does the answer change so much by month?

Because your use and the sun both change. Air conditioning is most of a summer bill in Texas and Florida, and summer days are also the longest and sunniest, so solar output rises with the load. Winter nights are longer, so the battery has more dark hours to cover. Test the month you actually worry about.

Can a battery run central air conditioning?

Sometimes, and size is only half of it. A battery has an energy rating in kWh, which is how long it lasts, and a power rating in kW, which is how much it can run at the same moment. A central AC compressor starting up can ask for more than a small battery can deliver. Ask the installer for the continuous and peak kW ratings and whether the system backs up the whole panel or only chosen circuits.

Do solar panels keep working in a power outage?

Only with a battery or a system designed to run off the grid. A standard grid-tied system with no battery shuts itself off when the grid goes down, so line workers are not hurt by power flowing back onto the wires. With a battery and the right equipment, the panels keep running the house and recharging the battery through the day.

How big a battery do you actually need?

With solar sized above your usage, size the battery for your night-time use in the month you worry about, with some room for a cloudy day. Without solar, start from how long your outages run: a battery that covers a night at half your normal use handles most evening outages, but no battery alone carries a whole house through days after a hurricane. For how often and how long each Texas utility's customers lose power, see outages by utility.

Where the subscription fits

The subscription systems we sell are generally sized to make more than the home uses in a year, where the roof has room for it. A home battery is available with the subscription in Texas, Florida and Illinois. To see what your own roof and bill would come to,run the estimator. Anything you are offered, from us or anyone else, you can read first with the contract checker.