When you install a home battery, what you are really doing is providing your home with a backup energy reserve in the case of an outage.

Whether you have a solar panel system at your home or not, a home battery can be used to store electricity and deliver it to your home appliances and devices. The benefits of a home battery are clear, especially during short or long-term outages, or in areas with Time-of-Use (TOU) rates that make electricity more expensive during peak use hours.

In this article, we’ll show you how to calculate how long a battery paired with solar can power your house during a power outage, and give you some tips for maximizing your battery usage.

First, the big question.

How long can a solar battery power a house?

Without running AC or electric heat, a 10 kWh battery alone can power the basic operations of a house for at least 24 hours, and longer with careful budgeting. When paired with solar, battery storage can power more electrical systems and provide backup electricity for even longer.

In fact, a recent study by the Lawrence Berkeley National Laboratory found that when heating and cooling are excluded:

“(A) small PVESS with just 10 kWh of storage (at the lower end of sizes currently observed in the market) can fully meet backup needs over a 3-day outage in virtually all U.S. counties and any month of the year.”

PVESS stands for photovoltaics and energy storage system.

But exactly how long you can power your home with solar battery storage varies for each home and depends on three main things:

Your battery storage capacity

The output of your solar system

Your electricity needs during an outage

We’ll show you how to budget your electricity to meet your solar and battery capacity below. First, let’s start with identifying your battery storage capacity.

Home battery capacity

Capacity — the amount of energy a battery can story — is one of the main features that influence how long a battery can power a house during a power outage.

Battery storage capacity is measured in kilowatt hours (kWh) and can vary from as little as 1 kWh to over 10 kWh. Multiple batteries can be combined together to add even more capacity, but a 10 kWh home battery is typical for most homes.

Check out this video to know more about Kilowatt hours (kWh).

During a power outage, assuming you have a fully charged home battery, you will be able to use most of the 10 kWh of stored energy. You’ll want to leave a minimum charge of 5-10% on your battery for a couple main reasons:

To maintain the health of your battery

To start your solar inverters in the morning so you can recharge your battery with solar power

For a 10 kWh battery, you’ll want to leave at least 0.5 kWh of capacity in reserve at all times. That leaves you with 9.5 kWh to power your home during a power outage.

On it’s own, 9.5 kWh of battery storage can power essential electrical systems for a day or two. But central air conditioning uses around 3 kWh per hour, which would eat up the entire battery in just over three hours. Let’s see pairing battery with solar can expand your options during a power outage.

Your electricity needs during a power outage

How long solar battery storage can run your home depends how much electricity you use. And how much electricity you use depends on which appliances and systems you’re running.

During a power outage, it’s recommended to budget electricity for the most necessary things first and then make a plan for the remainder capacity. For many homeowners, the list of priorities includes:

Refrigerator

Kitchen and cooking

Water heating

Lights

TV and device charging

Heating and cooling

Now, heating and cooling is last on the list not because it isn’t important, but because these operations take so much electricity they’ll likely have to be limited regardless of your battery capacity. So the wise move here is to budget your essentials first, and then get a sense of how much battery capacity is left for heating and cooling.

Note: If you have a gas furnace, you won’t need much electricity to keep the heat on.

Let’s run through an example scenario of powering essential systems during a 24-hour power outage to get an idea of how much solar and battery capacity you’ll need.

Use the tables below as an a la carte menu to create your own battery storage budget.