As soon as there is already a normal photovoltaic system, every extension must be registered and equipped with an additional meter. The yields are then billed separately.
but then it is no longer a balcony system, and the simplified registration no longer applies. It is then a completely normal photovoltaic system that must be approved in advance and installed by a certified electrician. If you can find one at all for the 2 modules, it will cost more than the savings of the next few years.
