Even though they usually include an inverter, most battery-based PV systems are “DC-coupled”—the PV array sends DC electricity to the system through a charge controller to the battery bank. That DC power is then drawn from the batteries for the loads, including through a battery-based inverter for AC loads.
AC-coupled PV systems are gaining wider acceptance and support through several battery-based inverter manufacturers. In AC-coupled systems, the DC power from the array is first converted to AC by a batteryless inverter, to be used by the AC loads through an AC load panel. Any unused energy is used by a separate battery-based inverter that either converts the AC to DC to charge the batteries, or, if it is a grid-tied system, it can also pass through to additional AC loads and/or the grid.
A batteryless inverter would normally not turn on without the utility grid present, but many battery-based inverters create a quality sine wave that’s good enough for the batteryless inverter to synchronize with. In grid-tied systems, when the grid goes out, the battery-based inverter isolates both the load subpanel and the batteryless inverter from the utility grid via an internal transfer switch allowing the batteryless inverter to remain on without being connected to the grid.
For residential systems, a primary advantage of AC coupling over the traditional system design is that you can add battery backup to an existing batteryless grid-tied PV system without changing the existing system’s wiring. An AC-coupled system can also be more efficient than a typical battery backup system because the batteryless inverter is doing the majority of the power conversion. Efficiency is generally in the 96% to 98% range compared to 90% to 95% for a typical battery-based inverter.
Disadvantages are a more complicated system to design and program, and more expense, since you’ll need two inverters (or more) instead of a single inverter and a charge controller. In off-grid use, AC-coupled systems are not self-restarting if the battery-based inverters happen to shut down because of low battery voltage. If this happens, the batteryless inverter does not sense AC voltage, and thus does not turn on to send array energy to the batteries. A DC-coupled system can self-restart even if the inverter shuts down from low battery voltage, because the charge controller can still charge the batteries.