How many solar batteries do I need?

Between falling battery prices, diminishing net metering programs, and skyrocketing electricity rates, more and more people are installing home energy storage systems. Adding battery storage to your solar panel system enhances your energy independence and overall savings––but to get the most out of it, you'll need an accurately sized system. 

The number of batteries you need depends on a few things: how much electricity you need to keep your appliances powered, the amount of time you’ll rely on stored energy, and the usable capacity of each battery. Given that the average solar battery is around 10 to 13.5 kilowatt-hours (kWh), most people need one battery for backup power, two batteries to avoid paying peak utility prices, and 10+ batteries to go completely off-grid. 

Every solar and battery setup is different, so it's important to consider your unique goals and needs when shopping around for solar and storage options.

The number of solar batteries you need depends on why you’re installing an energy storage system. Generally, people use battery storage systems for one of three reasons: to save the most money, for resiliency, or for self-sufficiency.

To save money

To save the most money with solar batteries, you need enough energy storage to keep your home self-sufficient during peak electricity pricing hours. Peak pricing hours vary by where you live and your exact utility plan. 

Typically, you’ll need about two average-sized batteries to avoid using grid electricity during peak hours and when your solar panels aren't producing power. You’ll still rely on the grid on a cloudy day, but you’ll be self-sufficient enough to maximize your solar investment.

For backup power 

Solar batteries are a reliable way to keep your home and essential appliances powered during extreme weather events and grid failures. 

A single battery will do the trick if you're only concerned with keeping a few things running during the average, quick outage.

To go off the grid

You’ll need around eight to 12 (or more) batteries to go off-grid. Self-sufficiency requires a lot of battery storage, especially if you build capacity for extra-long periods without sunlight (cloudy weather, nights, etc.).

Electricity loads

A "load" is anything that consumes electricity—like refrigerators, air conditioners, dishwashers, and other appliances around your home. You’ll need a good idea of how much electricity your appliances use to accurately size a storage system. 

There are a few ways to estimate the electricity load of your home’s appliances and systems:

• The Department of Energy’s (DOE) Appliance Energy Calculator. This tool uses the average power requirement of appliances to estimate average annual energy use and cost. 

• Energy monitoring systems provide insight into how much energy you use and when you use it. Home energy monitors are specific to your home, and some can even track usage at the individual appliance level. 

• Lastly, you can do it the old-fashioned way by reviewing your past electricity bills and looking for patterns in your peak usage. What appliances do you normally run at those times? This isn’t a perfect method, but it can give you some insight into your electricity loads if you’re willing to do the research.

Solar panel system size

The amount of power your solar panels produce determines how much they can charge your battery system during the day. It’s important to size both your solar panel and battery storage systems to work together; there’s no use in installing a huge battery if you’re never going to use its full capacity. Monitoring your solar panels’ production can help you understand how many solar batteries you actually need. 

Solar monitoring systems can provide insight into your system’s production and more. Monitoring systems are becoming increasingly available and robust, and most top manufacturers offer an easy-to-use app that is accessible right on your smartphone. 

Another way to monitor your solar electricity production is through a meter. If your solar panel system is grid-tied (most are), you’re likely familiar with this technology. Whether bidirectional or one-way, meters measure how much electricity your system produces. You can use this data to inform your battery system design. 

Your utility electricity rate

Electricity rates are either flat or variable. While the goal of battery storage is generally the same—pull as little electricity from the grid as possible—your motivation might be slightly different depending on your rate structure. 

On a flat-rate structure, you'll want enough storage capacity to rely on the grid as little as possible. The more you can store from your solar panels and use them later, the better your long-term savings will be. 

If you’re on a variable-rate plan like time-of-use or demand charge, you’ll want to ensure you have enough storage capacity to ride out the high-cost times of the day, at a minimum.

In some areas, you can participate in demand response programs as a battery owner. During peak demand times—like on a summer evening when everyone is running their ACs—the utility will pull electricity from your battery to relieve grid demand, and you’ll be compensated for it. It can be beneficial to install slightly more storage capacity with this program in mind, although it’s generally a seasonal gig with strict limits.

You’ll need to know a few things before you can calculate how many solar batteries you need to power your home. 

• Time (hours): The amount of time per day you plan on using stored energy. 

• Electricity demand (kW): From all of the appliances and systems you want to run during those hours.

• Battery capacity (kWh): The average solar battery is roughly 10 to 13.5 kilowatt-hours (kWh) in size.

Once you have these numbers, multiply the appliances' electricity demand by the number of hours they'll need to run. That’ll tell you the kilowatt-hour (kWh) capacity you require for storage. From there, determine how many batteries you need to meet those demands based on their usable capacity. 

Calculating electricity demand

First, calculate the electricity you use when you can't rely directly on your solar panels. Let’s break down an example together. 

Let's say you have a 1,500-watt (W) dishwasher, a 3,000-W air conditioner, an 800-W refrigerator, plus lights, WiFi, and miscellaneous appliances that consume 1,000 W of electricity. You need about 6.3 kilowatts (kW) of electricity from a storage system for the hour you run your dishwasher, and 4.8 kW of electricity the rest of the time. 

Calculating the amount of time you’ll use stored electricity

Your calculation depends on how you use your battery: