What is a solar battery?

Simply put, a solar battery is a device that stores the energy solar panels generate when the sun is shining and allows you to use it at another time. But what that really means is that a solar battery gives you much more control over the energy your solar energy system generates. It can also help protect you from weather outages and other blackouts.

When the sun is at its brightest, there’s a good chance that your solar panels are generating more electricity than you can use. When you store solar energy in a battery, you control how you use it. You can:

  • Send it into your home after the sun goes down
  • Keep it until you need it during a blackout
  • Send it to the grid at a time when the utility company is paying a better rate for it.*

Whatever you do, a solar battery puts you in control of the excess energy your panels produce.


solar battery powering up

Types of batteries and their chemistry

The most common solar battery types are lead acid and lithium-ion. What’s the difference? Lead acid batteries have been on the market for a long time and they have low DoD (Depth of discharge) as well as a short lifespan. However, they are the least expensive option. Lithium-ion is what most home solar batteries from leading manufacturers are made of. Lithium-ion has a higher DoD and lifespan, and is a lot more compact and lighter than lead acid.

Among lithium-ion batteries, Lithium Iron Phosphate (LFP) is the safest option, compared to Nickel Manganese Cobalt (NMC).

What does depth of discharge mean?

Depth of discharge (DoD) just means how much the battery can hold a charge before it needs to be
recharged. Like, your cell phone, for example. If you had to recharge it every time it said 5%, you would have a 95%

What this means for you: look for a solar battery that has a high DoD, ideally over 95%.

How does a solar battery store energy?

Although there are other types of solar batteries, the most common type of solar backup battery for the home is lithium ion. Lead acid batteries require a lot of maintenance and flow batteries aren’t practical for the home, so let’s focus on how our favorite technology, lithium-ion, is used by solar batteries to store energy.

Lithium Ion Batteries

Lithium ion, or li-ion, is the same technology that keeps your cell phone buzzing, your power tools running and your laptop working all day long. Li-ion batteries revolutionized tech when they started showing up in devices 30 years ago, but in the last decade or so, we’ve been able to power our cars and even our houses using Li-ion batteries.

Taking a dive inside your batteries

Li-ion batteries are highly engineered, but they work on a pretty straightforward principle. Each battery has a positively charged cathode and a negatively charged anode. Each of these is typically a liquid separated from the other by a semi-porous membrane.

When you turn on a device like your phone, you make a connection outside the battery from the anode to the cathode. That’s called a circuit. Electrons flow through that connection, creating an electrical current. Presto, you have electricity. But the electrons only flow because there is a chemical reaction happening inside the battery. Positively charged lithium ions cross from the cathode through the membrane to the anode, completing the circuit. The battery will keep running until most of the positive ions are in the anode. That’s when your phone dies.

ADT Solar electricians working on the inside of a solar battery

When you plug your phone in to charge it, the current is reversed, and the lithium ions flow back to the cathode and store up energy.

One of the reasons lithium makes such good batteries is that lithium ions are very small molecules, so they can flow through the membrane between the cathode and anode while the rest of the chemicals that make up the battery stay put.

For efficiency and safety, most large lithium-ion batteries are made up of a bunch of smaller batteries working together. Cellphone manufacturers, automakers, and solar backup battery manufacturers have developed sophisticated software to make them more efficient and last longer by rotating through those little individual batteries.

How does a solar battery work


What is battery coupling?

If you’re researching solar backup batteries, one of the technical questions you might read about is, “What type of coupling do you want to use?”

Rest assured, an ADT Solar Energy Specialist will take care of this for you if it seems too complicated. But if you want to know more, read on.

This is really just a question about where in the flow of electricity from your rooftop solar system to your home you want to connect your battery, but it has big consequences. Where in the sequence of components you put it determines how the battery will behave in your system.

Home battery storage installed on a wall

Types of coupling and what they mean for you

So, what are the different types of coupling? If you’re talking about electricity, AC or DC is almost always the first question. AC stands for alternating current while DC stands for direct current. The lights and appliances in your home run on AC. Batteries and solar panels run on DC. Getting everything to work together requires is what ADT Solar’s experts do best.

If you have solar panels on your home, the DC current they generate goes through an inverter that turns it into AC current before it flows into your electrical panel and then into your house. Most inverters are pretty efficient, so there’s not much energy lost by that change.

When you add a battery into the system, things get complicated. How the panels and batteries are connected is called coupling, and there are a couple options.

AC vs DC Coupling

AC stands for alternating current while DC stands for direct current. The lights and appliances in your home run on AC. Batteries and solar panels run on DC. Getting everything to work together requires is what ADT Solar’s experts do best.

DC Coupled Storage

With a DC coupled system, the current from your solar panels doesn’t require conversion before it’s stored. This is the most efficient way to get the electricity into the battery, but it requires more specialized wiring further down the process and that often makes it a more expensive option. For a DC coupled home, the energy generated on your roof has a more direct path to energizing your home:

  1. Sunlight strikes your solar panels and generates a direct current.
  2. The current flows to a charge regulator, which ensures that the amount of energy flowing to the battery isn’t more than it can handle
  3. The direct current electricity is stored in your lithium-ion battery
  4. The energy is released from your battery, passes through an inverter which converts it into alternating current, and enters your home’s electrical panel, where it’s sent to power your appliances and lights. Or, once it has passed through the inverter, it can be sent to the grid for credit if your utility offers that option.

AC Coupled Storage

With an AC coupled system, the DC current your panels produce is converted several times between AC and DC before it’s used. That doesn’t sound very efficient, but very little energy is lost when high-quality solar inverters are part of the system.

Also, using AC coupled storage means there’s more flexibility in your energy system. If you have an existing solar energy system, adding a backup battery is really only possible with AC storage, and you can also charge your backup battery using grid power. But storing your energy through AC coupling requires a more roundabout process:

  1. Solar panels convert sunlight to direct current electricity.
  2. The electricity flows into the solar inverter and is converted into AC electricity.
  3. AC electricity flows into your electrical panel and is used to power appliances and lights in your home.
  4. Any extra electricity is sent through another inverter and is converted back to DC electricity.
  5. The electricity is stored in the DC battery.
  6. When the energy is called into use, it again passes through an inverter where it changes again to AC and can be used in the home or sent to the grid.

If going solar or adding a solar backup battery seems complicated, it doesn’t have to be. ADT Solar has decades of experience designing and installing solar energy systems for all types of homes and we can handle any questions you have and make the whole process simple. If you’re ready to gain the peace of mind, security and control that comes from storing your own electricity in a solar backup battery, reach out to schedule your free solar consultation with one of our Solar Energy Specialists.

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* Net metering and similar excess or buyback programs vary by location and utility provider and are subject to change. Rates may go up or down and the money you may save, if any, may vary. For more information about rates, contact your power company.