Assessment of photovoltaic system, sizing

For yield optimization, you indeed do not need to look at solar tiles.

I am also a fan of good aesthetics, but my green conscience outweighed that.

In view of the many very different photovoltaic systems and materials used, this is an undifferentiated general statement about solar tiles and is of course not to be taken seriously in terms of content.

I would also ask you, how can I calculate from which kWp the Renewable Energy Sources Act surcharge becomes profitable

Approximately from 13 kWp

Regarding the area: take some photos, measure the roof (if necessary, measure and count tiles) and upload it on this other site. Try to layout the roof on paper yourself. A standard module is 1x1.7 meters.

Only when you know the number of modules, exact layout, and options for stringing/inverter would I continue with this or another solar installer.

I will definitely ask for advice in the photovoltaic forum following your recommendation, nevertheless (because I feel very well taken care of and advised), I would also like to ask you how I can calculate from which kWp the Renewable Energy Act surcharge becomes worthwhile. As silly as it sounds, I am really bad at math and therefore unsure how and with which variables to set up the calculation....

Basically, you have to compare the additional costs from the still payable Renewable Energy Act surcharge with the additional income: Your own consumption will typically be around 1000 kWh/year. On that, you pay IIRC about €0.05/kWh, so €50/year. Per kWp, after depreciation, you have about €30 to €40 profit per year. So to come out better overall, you should build at least 11.5, with a bit of safety rather the often recommended 13 kWp. The numbers can of course vary individually, but I hope this explains the procedure for the calculation.

When optimizing returns, you indeed don’t need to look at solar tiles.

Considering the many very different photovoltaic systems and materials used, this is a generalized blanket statement about solar tiles and, of course, should not be taken seriously content-wise.

Solar tiles have only a niche existence for good reason... Not just because of economic efficiency. Detailed discussion is available here in the forum.

You look good, the basic idea is a good one. The rest of the argumentation is hard to follow, since price does not play a subordinate role for most people. You can do all of that out of idealism.