This being Germany, you actually are supposed to register every panel in a central database. So the utilities know where generation is happening. This is for proper solar installs as well as for balcony solar.

I have a proper setup on my roof, and installed a 2kW balcony setup (2kWp panels mixed with an inverter limited to 800W) at my in-laws place.

Both are registered in the central database. I got a new power meter for mine. But it seems my in-laws are to keep their old power meter for a while, which occasionally just turns backwards, whenever they produce more than they consume.