A lot of folks believe that a solar power system will protect them from blackouts. While this is true of off-grid systems, grid-tie systems shut off when the grid goes down. AC Coupling allows a grid-tie system to work with a backup battery system that can be charged even during grid blackouts. It's perfect for folks who want to add a battery back-up package to their existing grid-tie solar power system.
In an AC Coupled solar power system, a grid-tied inverter can continue to operate during grid power outages by syncing with the output of a battery-based inverter. In this scenario, both inverters are connected to the "critical loads" sub-panel. A "critical loads" sub panel is a subsidiary breaker panel that's separate from a building's main breaker panel, designated only for appliances backed up during power outages.
Wholesale Solar designs and sells AC Coupled systems using Magnum Energy PAE series inverters, because they are the safest and most versatile inverter for this type of application.They will not backfeed to the grid, and they can be "coupled" with a variety of grid-tie inverters including microinverters.
How do these AC Coupling Systems work?
When the grid is functioning normally, the Magnum Energy inverter will use both the utility grid power and the AC output of the string inverter to maintain charge on the battery bank and to power the critical loads.
During utility blackouts, the Magnum off-grid inverter creates a "local grid" by supplying power to the critical loads in the subpanel. The grid-tie inverter doesn't disconnect because it doesn't know the difference between the utility produced grid and the "local grid" created by the Magnum inverter. When the grid is working, the Magnum inverter allows solar power produced by the gridtie array to feed back to the main utility service breaker panel. The Magnum Inverter is UL 1741 compliant which means it won't backfeed a dead power line.
In its AC-Coupled mode, the Magnum inverter will shift its frequency to 60.6 Hz to un-synchronize with the grid-tie inverter or back to 60.0 Hz to re-synchronize. The battery will re-sync and charge when the voltage falls too low, and unsync when the voltage rises too high to prevent overcharging.
Click image to enlarge diagram
The wiring diagram (at the top) is complex, but once you digest it, you will see that it is a unique method for wiring up a Magnum inverter. Read more.
Read More about AC Coupling.