Solar batteries come in two flavors, DC and AC coupled, which differ in how efficient they are and how they connect to your solar panels and the grid. If you’re adding a battery to an existing solar installation, it’s almost always more cost-effective to choose an AC-coupled battery. If you’re getting a new solar+battery installation, DC-coupled solar batteries are almost always more efficient and simpler to operate.

But what’s the difference between AC and DC coupling, and which one is best for you? Let’s find out.

How solar homes use AC and DC electricity

Direct current (DC) electricity means electrons flowing in one direction to provide power to a circuit. Alternating current (AC) electricity means electrons flowing in two directions, switching rapidly. The reason we have AC electricity is it’s cheaper and safer to transmit over long distances, and therefore many of your home’s appliances were built to use it.

Solar panels create DC electricity, which in a home solar system flows into a box called a solar inverter, which changes the DC electricity to AC electricity your home’s wires carry to outlets and appliances. Batteries also store and transmit direct current (DC) electricity, and they also need to be connected to an inverter, too.

Solar battery banks can add flexibility and peace of mind for homeowners. You can use them to back up your whole home in the case of a power outage, or choose just enough batteries to keep critical loads up and running.

DC-coupled solar batteries

When a battery is installed alongside a home solar system, the solar installer can install special equipment that first connects the solar panels to a charge controller, then to a battery bank, then a solar inverter, and finally to the home’s main electrical panel. This is called DC-coupled battery backup, because the battery is charged and discharged using DC electricity.

Some batteries commonly used in DC-Coupled systems include the 400V LG Chem Resu Lithium ion battery, SimpliPhi’s Lithium Ferrous Phosphate batteries and Trojan’s Flooded Lead Acid batteries.

Here’s a diagram of a DC-coupled solar battery system:

Advantages of DC-coupled batteries

• More efficient, because there is no conversion of DC to AC until after the batteries charge
• Makes it easy to charge the batteries from solar panels, even when the grid is down
• If the inverter goes offline, the solar panels can still charge the batteries because the charge controller and inverter operate independently
• Many popular DC-coupled batteries can be installed as AC-coupled systems by adding a battery-based inverter between the main panel and the battery

Disadvantages of DC-coupled batteries

• DC Coupling is not available for PV systems that use microinverters
• DC-coupled batteries are more difficult to add to an existing solar installation

AC-coupled solar batteries

For a long time, DC-coupled batteries were the only kind available, but more recently, people who already have solar panels needed an easy way to add battery backup in case of power outages and mandatory shut-offs.

For people who already have solar attached to a grid tie inverter or microinverters, a DC-coupled battery doesn’t usually make sense. Why install expensive new equipment that replaces what’s already working? Instead, current solar owners can add AC-coupled battery storage to an existing solar system.

Unlike a DC-Coupled system, an AC-coupled battery has its own inverter, which means it produces AC output that goes directly to your home’s main panel. Special wiring allows the battery to charge from the grid or your solar panels, and also maintain backup power if the grid goes down.

Popular AC Batteries include the Tesla Powerwall, Panasonic EverVolt AC, and the sonnen eco.

Here’s how an AC battery system all connects together:

Advantages of AC-coupled batteries

• Relatively easy to add to a home because they come with all the hardware necessary
• Can be added to a home with or without solar panels
• If something goes wrong with your solar panels or inverter, the batteries can keep working, and if something goes wrong with your batteries, the solar panels can keep working

Disadvantages of AC-coupled batteries

• Less efficient than DC systems, because the electricity needs to be converted from AC to DC for charging, then back to AC for use in the home
• Additional hardware means more parts that can fail and cause problems

There’s a lot more to learn, and if you want to see if a solar battery is right for your house, get the process started with our new solar panel calculator today!