Adding battery backup to a grid-tie solar power system via AC coupling (as opposed to DC coupling) means adding a new hybrid inverter, which takes over as the interface between your solar system and the grid. This allows either grid power or your solar power to charge your batteries (whichever is available), or even a generator in the case of an inverter with two AC inputs. When the grid goes down, the new inverter automatically disconnects your system from the grid and uses your solar or battery bank to power your critical loads - essentially operating as a code-compliant off-grid solar system.
The primary advantages of AC coupling vs DC coupling for battery backup are that you do not need to add a charge controller to the system, and all of your DC wiring can stay as is.
The primary disadvantage of AC coupling vs DC coupling is that battery charging is not as efficient, meaning your new inverter and/or battery bank will need to be a bit oversized compared to your actual power needs.
Amy Beaudet explains AC coupling in detail in this video.
AC coupling is almost always preferable to DC coupling for systems using microinverters or optimzers (like SolarEdge).
The AC coupling solutions below range from labor-intensive (like our MidNite kit, which includes neither the battery-based inverter nor the batteries) to plug-and-play (like the SimpliPhi AccESS units, which include everything pre-wired together in an enclosure). If you're unsure about how to best add battery backup to your grid-tie solar system, give our techs a call at 800-919-2400.