I don't think this is a Schuko limitation... 800W is a limit that you can send back to the grid without having a properly registered PV plant with a normal inverter. Your meter might disconnect you if you try to send more.
Yes, sorry. I think it is a battery policy regulation in Germany that prevents >800W.
He has 30 kW solar so the registration with the grid operator already happened.
This isn't a grid limitation but a rule about safe home installations. The limits are low for things the general public gets to plug in on their own. Those simple limits don't apply to the same battery installed by a professional. Professionals would instead follow a more complex set of rules and make some calculations, allowing for much higher currents if done in the right way.