FYI, Gangster Octopus, as soon as your system detects that the grid is down it will shut down, even if the sun is shining. It’s a common misunderstanding and very negative surprise, especially for people who put thousands up front into their system.
If it didn`t shut down, your system’s inverter would feed power into the grid and create a local island of power on the grid.
The primary problem with that is electrocuting linemen who expect to be fixing equipment on a grid with no power.
The other problem is that your inverter is designed to match the frequency and voltage of the grid, so without anything to match they’ll vary quite a bit and damage your neighbors’ electronics.