Off Grid Inverters or Grid Interactive Inverters

Grid interactive, or dual function, inverters can export power to the utility grid, but can also supply emergency backup power for critical loads during a grid outage. These inverters use a battery bank for energy storage (they will not operate without batteries), and include an automatic transfer switch that enables them to safely operate off-grid during a blackout.

The grid interactive inverter is connected to the battery bank (usually 12, 24, or 48 VDC), an AC subpanel for critical loads, and the building's utility entrance load center. The battery bank is charged by the PV array connected through a charge controller (see Charge Controllers). Under normal conditions, it operates like any grid-tie inverter and will export any excess power produced by the PV array. During a grid outage, the inverter will automatically disconnect from the grid, and supply AC power to the critical load subpanel by drawing energy from the battery bank.

When the outage is over, the inverter will automatically switch back to grid-tie operation and recharge the batteries. The Outback Radian GS and the Schneider XW grid interactive inverters have split-phase 120/240 VAC output from a single inverter. The Outback GFX and SMA Sunny Island inverters have 120 VAC output, but two inverters can be "stacked" for 120/240 VAC output.

The SMA Sunny Island is designed to interact with a Sunny Boy grid-tie inverter to create an "AC Coupled" power system. See the Sunny Island listing on page 102 for more information. There is some loss of system efficiency with grid interactive systems, compared to battery-less grid-tie systems.