Actually, that's not necessarily true. In a laboratory setting I have gotten a Sunny Boy, a Sunny Island, and a Generac generator to play nicely with each other in a small off grid system. Initially, the Sunny Island was acting as the grid and the Sunny Boy didn't know the difference. When the batteries dropped below the SOC trigger point I programmed in, the Sunny Island started up the generator and went into bypass mode, and the Sunny Boy kept running. The Sunny Boy recognized the power from the generator as grid and kept producing power to assist in charging the batteries and running the AC loads. When the batteries were topped off, the Sunny Island shut down the generator, went into inversion mode, and munged the frequency of its output to throttle back the Sunny Boy. It's a pretty cool system.
But please note that there are batteries and a battery inverter (Sunny Island) at the heart of the system. The Sunny Island prevents the Sunny Boy from backfeeding the generator.
