How long can a solar battery power your house?

Planning for backup power means figuring out what you'll need during an outage. A solar battery can keep your essentials running for about 24 hours, but the actual runtime depends on which appliances you consider necessary and how much power they use.

If you're researching solar batteries, you probably want to know how much of your house you can power and for how long. The short answer? A typical 13 kWh battery (the size of a Tesla Powerwall 3) can keep your refrigerator, lights, WiFi, phone chargers, and TV running for nearly a full day. But every home is different, and your battery's performance depends on your specific power needs and usage habits.

We'll break down what determines how long a solar battery can power your house, which appliances matter most during outages, and how to size a system that keeps your home comfortable when the grid goes down.

When discussing how much of your home you can power with a battery, the two main factors to consider are how much power you need and how much power your battery supplies.

To figure out these details, it's helpful to have a working knowledge of two common electrical terms: amps and kilowatts.

1. What appliances do you need to back up?

To determine how much power you need, you must know which appliances (or circuits) you plan to back up. Many homes in the U.S. have a 200-amp electrical panel. If you want to back up the whole electrical panel and simultaneously provide power to every circuit, you would need a lot of power. Most home batteries won't cover those needs.

Thankfully, it's unlikely that you'll be running every single appliance in your home on any given day, and you're even less likely to be running all of them at once. That means you (or your installer) will need to calculate the power usage of different appliances in your home or of different circuits on your electrical panel (most circuits can hold a maximum of 15 to 20 amps).

Batteries provide power ratings in kW and current ratings in amps, so if you know the power draw or current requirements of different appliances, you're in luck. If you don't, here are two resources to start with: First, our explainer on electrical load, and second, the U.S. Department of Energy's appliance load calculator.

Here's a breakdown of power consumption for common household appliances:

2. Your battery’s power rating (peak and continuous)

Once you know how much power you need to back up part or all of your home, you can begin to size an energy storage system appropriately.

There are two key power metrics to look at: peak power and continuous power.

Peak power determines if you can provide an extra surge of power to appliances that need it. For instance, a well pump or sump pump might require a lot of power when you first turn it on, but then its power requirements will drop for the rest of the time you're running it. If you have an appliance with a surge requirement for power when you first turn it on, look carefully at this spec.

Continuous power represents the amount of power (in kilowatts) your battery can provide steadily. This is the metric to determine how many different appliances and circuits you can power at once for hours at a time. WiFi routers and box fans are examples of appliances that require continuous power, but not much instantaneous power.

The most popular battery on EnergySage, the Tesla Powerwall 3, has a peak and continuous power rating of 11.5 kilowatts, meaning it can power most circuits concurrently.

When determining how long you can power your home with a battery, the primary factors to consider are the usable storage capacity of your battery, and which appliances you'll want to power and for how long. (Think long and hard if you need to power all those lights during an outage.)

But it's also important to consider whether your battery is paired with solar and if you are incorporating any load management systems along with your storage system.

1. Usable storage capacity of your battery

The first factor to know is how much electricity your battery stores. If you're looking at spec sheets or your storage quote (something EnergySage makes easy to do with our Buyer's Guide and our online comparison-shopping EnergySage Marketplace), the metric to look for is usable storage capacity. Usable storage capacity is listed in kilowatt-hours (kWh) since it represents using a certain amount of electricity (kW) over a certain amount of time (hours).

To put this into practice, if your battery has 13 kWh of usable storage capacity, you can either use 7.5 kilowatts of power for 2 hours (7.5 kW * 2 hours = 13 kWh) or 1 kW for 13 hours. As with your phone or computer, your battery will lose its charge faster when you use the device more.

2. Which appliances you're using and for how long

When you know your battery's usable capacity and your appliances' power consumption, the next step is to determine which appliances you plan to use and how long you'll be able to use them.

If your battery has a usable capacity of 13 kWh, you can power a:

You'll likely be running multiple appliances at once, which makes the backup calculation much more dynamic with many tradeoffs. For example, if you run your heat pumps for two hours, you’ll lose out on 20 hours of fridge time.  But if you plan to keep the essentials—phones, computers, WiFi, refrigerator, and some lights—running during an outage, you can expect a 13 kWh battery to keep those things running for over a day (about 34 hours).

3. Whether your battery is paired with solar

If you install a standalone battery, you won’t be able to recharge it until grid service is restored. So, if you experience frequent but short electricity outages, a standalone battery is a great way to keep your home running while your power is out.

However, if you experience longer-duration outages—reaching a day or more—a standalone battery isn't the right solution for backup power.

In that scenario, pairing a battery with a solar panel system is best. When you pair solar with storage, you can provide backup power to your home indefinitely, as long as the sun rises. Even if you have a cloudy day or two, once the sun starts shining in full again, you can recharge your battery and keep your home powered even if the rest of your block remains stuck in the dark.

Generally speaking, a battery with 11.5 kW of continuous power (like a Tesla Powerwall 3) will be able to power many different appliances at once: Kitchen appliances like a refrigerator (200 W), coffee pot (1,000 W), a microwave (1,200 W), and a dishwasher (1,200), electronics like a TV (100 W), computer (100 W), WiFi router (10 W), and phone charger (20 W), a dozen LED light bulbs (120 W), a washing machine (500 W), and a furnace fan for gas heat (600 W). However, if the battery is 13 kWh, it’ll only be able to run all of those devices at once for about 2.5 hours. 

Can you keep your home cool off of a single battery? Most batteries today can run a central air conditioner (about 3,000 W) for a few hours. But once you add additional appliances, your battery will drain pretty quickly. In the example above, if you swap the furnace fan for gas heat with a central air conditioner but continue to run everything else, your battery will only run for about an hour and 45 minutes. 

The longer you keep each appliance on, the less stored energy you'll have to power other appliances to get you through the night or until the next sunny day when you can recharge with solar. 

As you compare your battery options, check to see if the battery app (or an app from your inverter or smart electrical panel) will tell you how much battery life you have left under different usage scenarios and conditions.