What’s the best coupling choice for solar battery?

Solar batteries are turning out to be increasingly popular. There comes out two choices when solar panels, batteries, and inverters are all connected: alternating current (AC) coupling and direct current (DC) coupling. Both have pros and cons that are depending on the specs of installation.

AC vs. DC coupling: Differences

The main difference between an AC-coupled and DC-coupled system is the path electricity takes after produced by panels. Solar panels generate DC electricity to be transformed into AC electricity for your home’s appliances. However, solar batteries’ electricity storage is in DC form.

In an AC-coupled system, DC solar electricity flows from solar panels to a solar inverter that transforms the electricity into AC electricity. That AC electricity can then flow to your home appliances, or go to another inverter that transforms the electricity back to DC for storing in a battery system. With AC-coupled systems, any electricity that is stored in the battery system needs to be inverted three separate times before use.

AC-coupled battery system 

In a DC-coupled system, DC solar electricity transmits from solar panels to a charge controller. It feeds into a battery system, which means there is no inversion of solar electricity from DC to AC and back again before storage in the battery. Any electricity produced by the solar panels will be inverted only once (from DC to AC), either as it flows from batteries to your home electrical setup or out to the electrical grid.

DC-coupled battery system 

Historically, AC-coupled battery storage setups have been more common for residential and commercial solar installations. But as more DC options become available, DC coupling is becoming popular.

Pros and cons of AC coupling

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

An additional benefit of AC-coupled systems is that these setups allow batteries to charge from both solar panels and the grid. This means if your solar panel system isn’t generating enough electricity to fully charge your battery, you can still rely on the grid to fuel your battery for resiliency or electricity rate arbitrage benefits. This is also an important advantage if you’re hoping to participate in a utility energy storage pilot or another type of demand response program, as your electric utility may need to be able to control the flow of electricity in and out of the battery.

As far as drawbacks are concerned, AC coupling means that stored solar electricity will need to be inverted three separate times before being used by home appliances. The process of inverting electricity from AC to DC, or from DC to AC, results in small efficiency losses, so the more inversions that take place, the larger the overall reduction in system efficiency. Most inverters have efficiency losses of about a few percentage points.

Pros and cons of DC coupling

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 separate times in an AC-coupled storage setup, DC systems convert electricity from solar panels only once, leading to overall higher system efficiency.

Which solar plus storage system is better?

If you already have a solar panel system installed on your property, an AC-coupled system is likely best for you to add more storage This is because 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.

If you’re installing a solar panel system and battery simultaneously, a DC-coupled system may be the better choice due to the higher overall efficiency of DC-coupled setups. However, the installation for DC-coupled systems usually takes longer, so it’s important to factor in the impact of your upfront installation costs.

Whether you choose any of them, installing a solar plus storage system on your property can be a great way to save money.