Most of the inverters require input power to function, for safety reasons, I suppose. However, I install a two pole contactor with the coil powered by the normal source. If utility power drops off line, the contactor de-energizes and disconnects the invertor from the grid. I don't like to rely on electronics to keep people safe. There is also the issue of a disconnect at the point where the power enters the structure and labeling indicating multiple sources of power at the panelboard.
Isn't the inverter in parallel with the utility, though? It seems as though the contactor would be self-sustanining, with the inverter powering its own coils even if utility power is lost. You couldn't even use directional CTs, as loss of utility power would be indistinguishible from selling power back to the grid.
Does this method rely on the impedance of backfeeding the grid to be so low that internal resistance depowers the coils below the sustainment threshold?
