At the heart of this DC coupling solution is a 600V Morningstar TriStar MPPT charge controller with a DC transfer switch that lets you switch from your existing grid-tie inverter to the backup battery bank included with this kit with the flip of a switch to keep your critical loads powered up when the grid goes down.
With this setup, the TriStar MPPT controller keeps the batteries charged while the grid is down. When the grid comes back online, simply flip the transfer switch back to its original position and you'll be operating as a grid-tie solar system once again.
One great benefit of retrofitting your grid-tie PV system with this Morningstar DC coupling solution is that you can oversize the PV array without harming the charge controller. Even though the max array input power for the controller is listed at 3.2kW, there is no problem using a 6kW or even a 7kW array to it because the TriStar limits any extra input power to provide its rated current output to the batteries. While not beneficial on sunny days, oversizing the PV array like this is perfect for cloudy days when each individual solar panel is generating less power and a larger number of panels is needed to get full power to the batteries. You can NOT oversize the array like this in an AC coupling solution, where the max array power is usually limited to 85% or 90% of the inverter power.
Another great benefit of Morningstar's DC coupling battery retrofit system is that it's scaleable. Start with the included 178 amp-hour battery bank for your most critical loads, and add batteries and inverter capacity in the future if needed.