AC vs. DC solar battery coupling: What you need to know

Solar batteries can provide financial savings, the ability to keep the lights on during utility power outages, and can even enable you to go off-grid–so it's no surprise that battery storage systems are becoming popular additions to solar energy projects of all scales. 

Regarding the configuration of your solar panels, batteries, and inverters in your home energy system, there are two main options: alternating (AC) and direct (DC) coupling. AC and DC coupling have advantages and drawbacks, so that the best system will depend on your needs and the specifics of your solar + storage installation.

Solar panels generate DC electricity that must be transformed (via inverters) into AC electricity, the type of electricity used by most of your home's appliances. Solar batteries store electricity in DC form. So, the difference between AC-coupled and DC-coupled batteries lies in whether the electricity generated by your solar panels is inverted before or after being stored in your battery. 

In an AC-coupled system, DC power flows from solar panels to a solar inverter, transforming it into AC electricity. That AC power can then flow to your home appliances or go to a battery inverter that converts the electricity back to DC for storage. With AC-coupled systems, any electricity stored in the battery system must be inverted three times before use.

In a DC-coupled system, DC solar electricity flows from solar panels to a charge controller that directly feeds into a battery system, meaning there is no inversion of solar electricity from DC to AC and back again before the battery stores the electricity. Any electricity the solar panels produce will be inverted only once (from DC to AC) as it flows from batteries to your home appliances or the electrical grid.

Historically, AC-coupled battery storage setups have been more common for residential and commercial solar installations. As more DC options become available, DC-coupled solar batteries are gaining popularity.

A solar panel's efficiency measures its ability to convert sunlight into solar energy, and a solar inverter's efficiency measures how well it converts this solar energy into usable AC electricity. For batteries, it's a bit more complex. 

Most solar batteries are rated on roundtrip efficiency, which measures how much electricity is lost when you charge and discharge them. The higher the efficiency percentage, the more efficiently the battery can convert incoming electricity into stored electricity and back into usable electricity. For example, if you send 10 kilowatt-hours (kWh) to your battery for storage and can take 9 kWh out for usage, your battery has a roundtrip efficiency of 90%.

The main advantage of AC-coupled solar battery systems is their ease of installation when retrofitting storage to an existing solar system. Easier installations require less labor and time for solar installers, which often means a lower upfront cost.

Additionally, both solar panels and the grid charge batteries in AC-coupled systems. Being grid-tied means that if your solar PV system isn't generating enough electricity to charge your battery at any point fully, you can still rely on the grid as a charger. This advantage is important if you're hoping to participate in a utility energy storage pilot or another type of demand response program, as your utility company may need to be able to control the flow of electricity in and out of the battery. 

The main disadvantage of AC coupling is that inverting electricity from AC to DC or from DC to AC results in small efficiency losses. AC coupling means that stored solar electricity must be inverted three times before being used by home appliances, leading to lower efficiency.

DC-coupled solar energy systems have the advantage of being more efficient than AC-coupled systems. While solar electricity is converted between AC and DC three times in AC-coupled battery systems, DC systems convert electricity from solar panels only once, leading to higher efficiency. 

That said, DC-coupled options are more complicated to install for retrofit storage systems, which can drive up upfront costs and installation time. DC-coupled batteries also may not qualify for specific utility storage programs that could help you earn money.

If you already have a home solar array installed on your property and want to add an energy storage system as a retrofit, an AC-coupled system is likely best for you: You'll already have a solar inverter system installed with your panels and rewiring for a DC-coupled system is a complicated process that can increase installation costs.

However, if you're installing solar panels and a battery storage system simultaneously, a DC-coupled system may be the better option because of the higher overall efficiency of DC-coupled setups. However, installing DC-coupled systems is usually more complicated, so it's important to consider how that will impact your upfront installation costs.

Whether you choose an AC- or DC-coupled system, installing solar plus storage on your property can be a great way to save money while generating and storing renewable energy. EnergySage is a free service that delivers a simple and transparent shopping experience to take the guesswork out of going solar. Request quotes from vetted installers and connect with expert Energy Advisors in minutes when you sign up on the EnergySage Marketplace today!