You cannot buy a PV inverter in Germany (entire EU really) that doesn't automatically shut off if it doesn't detect a frequency to sync against from it's AC side, unless it can run off-grid in which case it has to disble the grid connection within the same 20ms.
So you can't buy a grid-connected inverter with off-grid capabilities? Because the inverter has no way to tell the difference between the grid being off, and being off-grid.