How a Solar Battery Works: Energy Storage Explained

Ready to take control of your home's power? A battery paired with home solar panels can make your home more resilient, stretch your solar energy further, and, in some cases, maximize your savings. 

This explanation of solar batteries and energy storage can help you decide if adding a battery to your home is the right choice for you.

What is a solar battery?

A solar battery is an energy storage device you can add to your solar power system to store electricity generated by your solar panels that you won’t immediately use.

You can use the stored energy to power your home at times when your solar panels don't generate enough electricity, including nights, cloudy days, and during power outages.

If you don't have battery storage, all excess electricity from solar power goes to the grid. In some locations this might be the most economical way to use your solar energy. In others — or if your goal is to use as much of your solar power as possible — a solar battery can be a good fit.

The science of solar batteries

Lithium-ion batteries are the most popular form of solar batteries on the market.

When you charge the battery, external electricity from your solar panels or the grid forces lithium ions to leave the cathode (positively charged) and move through a liquid electrolyte to the anode (negatively charged). It’s a bit like pushing a ball up a hill; you are actively storing potential energy in the anode.

Once charged, the ions stay in the anode until the battery is called on to release some electricity. The chemical energy sits ready for use, held in place by a physical separator.

When you draw electricity from the battery, the "ball" rolls back down the hill. The ions naturally flow back to the cathode. As they move internally, they release electrons that also want to flow back to the cathode. They can’t cross the separator, so they flow through the wires of the device or home the battery is powering. This is the electrical current that powers your home.

Home solar power storage batteries combine multiple ion battery cells with sophisticated electronics that manage the performance and safety of the whole solar battery system. 

Comparing battery storage technologies

When it comes to solar battery types, there is one obvious leader, with several other technologies filling niche cases or emerging in the market.

Lithium-ion battery

Battery manufacturers prefer lithium-ion battery technology for its higher depth of discharge, lengthier lifespan, ability to hold more energy for longer, and more compact size. These benefits come with a higher price tag than lead-acid batteries.

Depth of discharge (DoD) is the percentage to which a battery can be used, related to its total capacity. If a battery has a DoD of 95%, it can safely use up to 95% of the battery’s capacity before it needs to be recharged.

Lead-acid battery

Lead-acid batteries (the same general technology as most car batteries) have been around for years, and have been used widely as in-home energy storage systems for off-grid power options. While they are still on the market, their popularity is fading due to lower DoD and shorter lifespan.

Solid state battery

Solid state batteries are the new emerging battery technology. They replace a liquid electrolyte with a solid one. This can make them lighter, thinner, and more energy dense. They aren’t yet widely available.

Sodium-ion battery

Sodium-ion batteries are more established than solid state batteries, but still relatively new. They could be cheaper, faster charging, and more durable batteries that challenge lithium-ion technology in stationary storage settings, like homes. They’re also built using more abundant materials than most lithium-ion batteries.

AC-coupled storage vs DC-coupled storage

Coupling refers to how your solar panels are wired to your battery storage system, and the options are either direct current (DC) coupling or alternating current (AC) coupling. The main difference between the two lies in the path taken by the electricity the solar panels create.

Solar panels create DC electricity and solar batteries store and discharge DC electricity. However, DC electricity must be converted into AC electricity before it can be used by your home. So there are different ways of connecting a solar battery, your solar system, and your home.

DC-coupled storage

With DC coupling, the DC electricity created by solar panels flows through a charge controller and then directly into the solar battery. There is no change between DC and AC before storage. That change only occurs when the battery sends electricity to your home, or back out into the grid.

A DC-coupled storage battery is more efficient, because the electricity only needs to change from DC to AC once. However, DC-coupled storage typically requires a more complex installation, which can increase the initial cost and lengthen the installation timeline.

AC-coupled storage

With AC coupling, DC electricity generated by your solar panels goes through an inverter first to be converted into AC electricity for everyday use by appliances in your home. That AC current can also be sent to a separate inverter to be converted back to DC current for storage in the solar battery. When it’s time to use the stored energy, the electricity flows out of the battery and back into an inverter to be converted back into AC electricity for your home.

With AC-coupled storage, electricity is inverted three separate times: once when going from your solar panels into the house, another when going from the home into battery storage, and a third time when going from battery storage back into the house. Each inversion results in some efficiency losses, so AC-coupled storage is slightly less efficient than a DC-coupled system.

Unlike DC-coupled storage that only stores energy from solar panels, one of the big advantages of AC-coupled storage is it can store energy from both solar panels and the grid. This means even if your solar panels aren’t generating enough electricity to fully charge your battery, you can still fill the battery with electricity from the grid to provide you with backup power, or to take advantage of electricity rate arbitrage.

It’s also easier to upgrade your existing solar power system with AC-coupled battery storage, because it can just be added on top of an existing system design, instead of needing to be integrated into it. This makes AC-coupled battery storage a more popular option for retrofit installations.

How solar batteries work with a solar power system

This entire process starts with the solar panels on the roof generating power. Here is a step-by-step breakdown of what happens with a DC-coupled system.

1. Sunlight hits the solar panels and generates DC electricity.
2. Electricity enters the battery and is stored.
3. DC electricity leaves the battery and an inverter converts it into AC electricity the home or the grid can use.

The process is slightly different with an AC-coupled system.

1. Sunlight hits the solar panels and generates DC electricity.
2. Electricity enters the inverter to be converted into AC electricity the home can use.
3. Excess electricity flows through another inverter to change back into DC electricity that can be stored.
4. When the energy stored in the battery is used, that electricity flows through the inverter again to become AC electricity.

How solar batteries work with a hybrid inverter

If you have a hybrid inverter, a single device can convert DC electricity into AC electricity and AC electricity into DC electricity. As a result, you don't need two inverters in your photovoltaic system: one to convert electricity from your solar panels (solar inverter) and another to convert electricity from the solar battery (battery inverter).

Also known as a battery-based inverter or hybrid grid-tied inverter, the hybrid inverter combines a battery inverter and solar inverter into a single piece of equipment. 

Hybrid inverters work with and without battery storage. You can install a hybrid inverter into your battery-less solar power system during the initial installation, giving you the option of adding solar energy storage down the line.

Benefits of solar battery storage

Adding battery storage for solar panels can help you get the most out of your solar power system. Here are some of the main benefits of a home solar battery storage system.

Stores excess electricity generation

Your solar panel system often produces more power than you need, especially on sunny days when no one is at home. If you don't have solar energy battery storage, the extra energy will be sent to the grid. If you participate in a net metering program, you can earn credit for that extra generation, but in places you don’t receive a 1:1 ratio for the electricity you generate, batteries may be an avenue to greater savings.

With battery storage, the extra electricity charges up your battery for later use, instead of going to the grid. You can use the stored energy during times of lower generation, which reduces your reliance upon the grid for electricity.

Provides relief from power outages

Since your batteries can store the excess energy created by your solar panels, your home will have electricity available during power outages. (In some locations, batteries may be installed as arbitrage, or consumption-only, systems. These are cheaper to install and let you store energy for later use, but don’t provide backup power.) 

Reduces your carbon footprint

With solar panel battery storage, you can go green by making the most of the clean, renewable energy produced by your solar panel system. Replacing more of your grid electricity with clean electricity from your roof will reduce your carbon footprint.

Provides electricity even after the sun goes down

When the sun goes down and solar panels aren't generating electricity, the grid steps in to provide needed power if you don’t have battery storage. With a solar battery, you’ll use more of your own solar electricity at night, giving you more energy independence and helping you keep your electric bill low.

Provides quiet backup power

A solar power battery is a quiet backup power storage option. You get maintenance-free clean energy, without the noise from a gas-powered backup generator.

When battery storage and going solar make sense

A solar battery can help you use more of the solar electricity you generate, provide resilient backup power, and, in some cases, boost your solar savings. But, it may not be the reight fit for your home.

Having the right system design is vital to making the most of your solar panels. A system that’s designed, installed, serviced, and maintained well could help you lower your energy bills for decades to come. Reach out today to get a free estimate of your savings.

Frequently asked questions

Is it worth getting batteries for solar?

Batteries for solar energy storage can be a valuable complement to your solar panels. Having battery storage lets you use solar power 24/7, maximize savings from your system, and have reliable backup power during grid outages.

How many batteries do you need to run a house on solar?

This depends on your needs and how you expect to use your energy system. Most home battery installations are wired to backup only emergency needs, and one battery is all that’s necessary. For owners going off the grid or needing to back up much more than just the essentials, stacks of multiple batteries may be needed.

How long does a solar backup battery last?

Major manufacturers often extend 10-year warranties for their batteries. You may be able to maximize your battery’s lifetime with smart usage habits.

How much does a solar battery backup system cost?

The cost of solar batteries varies quite a bit depending on the capacity and number of batteries you need and the incentives, such as tax credits and rebates, available to you. Home solar batteries can cost up to $15,000 to purchase and install.

Disclaimer: This content is for educational purposes only. Palmetto does not provide tax, legal, or accounting advice. Please consult your own tax, legal, and accounting advisors.