[the-unknown-dev]

This is not really an issue. The inverters used in these cases rely on an existing power supply for synchronization with the electrical grid. If there is no power on the socket, the inverter has no means to generate a sine wave on its own and does not provide power on its cable.

This is true (as far as I know) for ALL inverters, apart from "off grid" solutions, so workers can turn off power on a street level. If that wasn't the case, there would be no way for technical staff to make sure that lines going to multiple homes (with solar) are powered off.

[davrosthedalek]

I can how it would not switch on if line power is gone. But what if it is on already, there are not many loads on, and the mains go down? Does it switch off? How does it know?

[bluGill]

It watches the incoming AC waveform and matches that. If there is no waveform it has nothing to match and so it shuts off.