Unfortunately that's the current EU law (EN 50549-1 / VDE-AR-N 4105).
Alternative power sources, when connected to the grid, must be non-aisled, i.e. they must automatically shutdown in a power grid failure.
The offical reason for this is to avoid accidents when the lines are under repairments in a power outage. But that's bullshit because there are technical solutions to overcome that issue like the automatic switch off systems, already legalized in the USA and many other places, that allow safe aisled installations coexisting with the grid that still power up houses on a grid outage.
The real reason is that the EU burocrats do not want autonomous power installations because that will jeopardize the elite's control over the population, and Big Energy will lose cheap electricity inflows.