Why not purchase a grid/hybrid inverter from the start? For those staring at their roof full of PV modules when the grid goes down, fully inoperable while the hours without power turn into days, and sometimes even weeks, that very same question is probably being asked many times over. Some may not have realized they would be without the PV power on their roofs when they bought their grid-tied system. Maybe they thought it would never happen to them. Finally, the relatively modest extra cost of a smarter grid-interactive inverter/charger and batteries may have initially discouraged them.
Adding energy storage through AC-coupling: For the owners of these more common grid-tied, grid-dependent inverters, there is a way to tie in a battery-backup inverter system using a method called AC Coupling. It typically requires adding a load center with circuit breakers and electrical connections for the building’s critical loads. This allows a point at which the grid-tied (GT) inverter and the battery-based (BB) inverter to “couple” and share their energy to the loads. In a normal mode of operation with grid power available, the energy from the PV array flows through the GT inverter to the critical load panel, with any excess energy flowing through the load panel to the BB inverter, and on out to the grid (see Figure 3).
When grid power is lost, the BB inverter activates an internal transfer switch which opens its connection to