AC Coupling
A storage architecture in which the PV inverter and the battery inverter are separate units connected on the AC side. Solar energy is converted to AC first, and any surplus is then converted back to DC to charge the battery.
AC coupling is the natural choice when adding storage to an existing PV installation, because the original inverter and array stay untouched. The cost is an extra conversion round trip for energy that goes into the battery, which loses a few percent compared with DC coupling. It also gives useful flexibility: the two inverters can be sized independently and sourced from different manufacturers. In backup mode the battery inverter forms the local grid and the PV inverter follows it, which is why frequency shifting is used to tell the PV side to reduce output when the battery is full.