Solar / Photovoltaic from an economic perspective

Exactly, that is added as well.

Hmm, so far I don't see any significant disadvantages of solar thermal... everything that has been said also applies to photovoltaics...

If the compensation for the electricity eventually disappears, it will look the same for photovoltaics...

In [D] neither is simply worth it...

Hmm so far I don't see any significant disadvantages of solar thermal... everything that has been said also applies to photovoltaics...

If the compensation for electricity disappears at some point, it looks the same for photovoltaics...

In Germany, neither is simply worthwhile...

Somewhat generalized judgment and presumably you don't have either.

Without wanting to make a big calculation now, a photovoltaic system pays off after about 12-15 years - depending on the agreed feed-in tariff, the interest rate, the repayment rate, the achieved self-consumption, and the current electricity price.

You are right - the fixed feed-in tariff disappears after 20 years. However, the system (hopefully) continues to produce - even if it is fully depreciated, you continue to feed in (additionally, possibly lower returns) and also continue to use self-consumption.

I do think that the system will pay off - but not yet after e.g. 8-10 years.

However, the system will then (hopefully) continue to produce – even if it is fully depreciated, you continue to feed in (possibly additional, albeit lower, yields) and also continue to use self-consumption.

The efficiency of the system decreases over the years, furthermore, if the inverter breaks down once, the whole naive calculation is at risk.

That's what I mean... and that the inverter and the cells are due after a little more than 10 years is practically certain... there are plenty of examples

In a single-family house, common solar systems whether photovoltaic or thermal are completely unprofitable and you at best break even...

Of course, if you have a factory roof or something similar available, it looks completely different...

How would it behave if solar thermal energy is also applied to the underfloor heating? In winter, it will bring no benefit. But the transitional period should be covered with it.