[rhodozelia]

It is a more complex and expensive inverter that can operate both isolated from the grid and in sync with it I think because it has to know when to do which and if it is outputting power how can it tell if the system is live because it is grid tied or because it is keeping the system energized itself.

I think they need external signals to tell them which mode to operate in.

Maybe the external system sees the frequency and voltage dropping or lower than normal range and decides to disconnect the load from the grid and opens a breaker and tells the inverter to operate in isolated mode.

When the external system sees the utility power is good again it could tell the inverter to shut off, close the breaker to connect the load to the utility, and then the inverter can go back to grid tie. The outage can be very short unless the load is large motors they don’t like the instantaneous change in phase when switching between different sources that are out of sync.