Construction costs are currently skyrocketing

(...)Because surpluses from balcony power plants also go into the general grid.

And indeed at 80% 1 to 1, because during the sunny hours of the day hardly anyone is at home.
ALGII recipients rarely have balcony power plants...

(...)
Roof systems are something completely different. Photovoltaics pay off more or less through the feed-in tariff and then become worthwhile through self-consumption. Here you are also able to operate something larger, e.g. hot water through a heat pump, electric car,...

At the current prices for photovoltaics, nothing pays off anymore. At least I have not seen any calculation that would even remotely prove this. Usually, the annual value is taken, averaged over 12 months, and then they say "Here you see. THIS is what you have for the heat pump in winter! WOW!" It just does not correspond to reality...

Well, I work from home and have specifically oriented one panel to the east and one to the south. Yesterday it was relatively sunny here and from 9 a.m. to 2 p.m. about 1800 watts were produced. The peak was 555 watts for a few minutes ... currently it still works with partial shade from the house on the other side of the street due to the sun being quite low. I am already looking forward to summer to see how the power output curve will behave then.

...
I once knew someone who was forced to undergo an audit by the tax office.
...

A few years ago, I had an audit. Two men from the tax office showed up with really large briefcases (plural!). That made me nervous... A few hours later, they were genuinely amazed that I had reported the rather unusual circumstance (only that was audited) 100% exactly, and they left. They are (mostly) only human too. But I also know opposite stories (forced to file a tax for a type of company that was not taxable at all. I believe more than 10 years later, it was judged and hundreds of thousands of euros were returned to those wrongly convicted. The company, of course, did not survive ... :-(

But who is supposed to check that...
If Aman simply states 600 watt inverter and has 800 watt?

Many metering point operators require the unit certificate of the inverter directly upon registration.

And indeed 80% one to one, because hardly anyone is at home during the sunny hours of the day.

Yes and no. You should try to orient one photovoltaic module to the east and one to the west. You have a lower annual yield, but it is better for covering the base load. At my parents' place, I installed the modules a few months ago on the shed with an east/west roof.

How do you actually use such a balcony power plant? Purely as an off-grid system via the inverter? Or is it connected with the apartment? Without storage, I imagine the use is difficult, maybe for charging a handheld device..