How do solar batteries work?

With interest in energy storage technologies on the rise, it's good to get a feel for how energy storage systems work. Knowing how energy storage systems integrate with solar panel systems–as well as with the rest of your home or business–can help you decide whether energy storage is right for you.

Below, we walk you through how energy storage systems work with solar and what that means for what you can expect to get from your storage system. We also take a more technical look at what's happening inside your battery to store that energy.

At the highest level, solar batteries store energy for later use. If you have a home solar panel system, there are a few general steps to understand:

1. Solar panels generate electricity from the sun

2. This direct current (DC) electricity flows through an inverter to generate alternating current (AC) electricity

3. The AC electricity powers your home appliances

4. Extra electricity not used by your appliances charges your batteries

5. When the sun goes down, your appliances are powered by the stored energy in your battery

It’s first worth a quick refresher on how solar panel systems work to understand how storage works with solar panels. 

Typically, when you install solar panels, you'll install a grid-tied, net-metered solar panel system. This means that when your solar panels produce more electricity than you need, you can return that excess electricity to the grid. Conversely, you can pull electricity directly from the grid when you use more electricity than your panels produce. Net metering allows you to run your electricity meter in reverse when you put extra energy onto the grid and run it forward when you pull from the grid, with your utility billing you for the net electricity you use.

With a solar plus storage system, you can use that electricity to charge your energy storage system instead of exporting excess solar production to the grid. Then, when you're using electricity after the sun's gone down, you can draw from your solar battery instead of from the electric grid.

When you install a battery with your solar panel system, you can pull from either the grid or your battery, when it's charged. This has two major implications:

Backup power

Even though you'll still be connected to the grid, you can operate "off-grid" since pairing solar plus storage will create a little energy island at your home. So in the event of an outage, either due to extreme weather or a utility shutoff, you'll still be able to keep your lights on. 

Two things to note about backup power. First, if you just have a solar panel system without a battery, you will not have power in the event of an outage, even if it's a sunny day. This is because your solar panel system will shut down in the event of a power outage so that it doesn't send electricity onto transmission lines while utility workers are attempting to fix them, which would pose a safety risk. 

Second, most batteries only provide backup power for part, not all, of your home. Unless you also install a smart electrical panel with your battery (which is a great way to get the most out of a storage system), most battery installations will require you to select what parts of your home you want to back up with the battery and pull those loads onto a critical load panel. However, many batteries can be "stacked", meaning you can keep adding additional batteries until you have the storage capacity you want. So while it might be possible to achieve whole-home backup, it can be cost-prohibitive to purchase enough batteries to provide that level of backup.

Avoid high utility rates

By allowing you to pull from your battery instead of from the electric grid, pairing a storage system with your solar panels can help you avoid high utility rates. There are two ways batteries can do this. First, if you are on a time-of-use or other time-varying rate, you can pull from your battery at the times when your utility charges more for electricity, i.e., during peak hours. And, second, if you are on a rate with a demand charge, which is more typical for commercial and industrial companies than for homeowners, a battery can help you lower your demand charge each month, which is a significant financial benefit.

Now that you know how storage works with solar, you are perfectly well equipped to decide whether or not to add storage to your solar panel system. If you’re interested in learning about the specifics of how batteries store energy, read on. 

Lithium-ion batteries

The most typical type of battery on the market today for home energy storage is a lithium-ion battery. Lithium-ion batteries power everyday devices and vehicles, from cell phones to cars, so it's a well-understood, safe technology. 

Lithium-ion batteries are so called because they move lithium ions through an electrolyte inside the battery. Since ions are particles that have gained or lost an electron, moving the lithium ions from an anode to a cathode produces free electrons, i.e., electrons released from lithium atoms. The build-up of these free electrons is how batteries ultimately charge and store electricity. When you discharge the electricity stored in the battery, the flow of lithium ions is reversed, meaning the process is repeatable: you can charge and discharge lithium-ion batteries hundreds or even thousands of times.

Lithium-ion batteries used in home energy storage systems combine multiple lithium-ion battery cells with complex power electronics that control the performance and safety of the whole battery system. Different types of lithium-ion batteries use slightly different chemistries to offer varied attributes, from improved power density to longer lifetimes. 

Notably, lithium-ion batteries aren't the only type of battery used in energy storage applications at the home, business, or utility level. The other types of batteries store energy via similar mechanisms, with an entirely separate set of pros and cons.

Here's why batteries are increasing in importance at a much larger scale: