Gas condensing boiler, air-water heat pump, fuel cells - please advise

solar thermal energy never pays off and also brings no increase in comfort. If possible, it can therefore be omitted.

Hmm, I can't sign off on that.

solar thermal energy never pays off and also does not bring any comfort gain. So if possible, it can be omitted.

Incredible, so many facts in two sentences.

Someone's feeling a bit offended...
Depending on the household size, you need about 1500 kWh of energy per year for hot water preparation.
At a gas price of 5 cents, that's €75, you say you achieve 75%, ergo €56 savings per year.
An ST currently costs at least €5000 with everything included, if you build with an all-in general contractor even more.
So, you have your investment back in 80-90 years, without interest. In those 80-90 years, the system will have broken down 5 times and you will probably be dead already.

I will just leave the values in your calculation as they are. But there is not only Max Mustermann, who by the way doesn't get offended that easily :-)

And we had to choose either solar thermal energy or insulation to comply with the energy saving ordinance. Since I don’t like insulation boards on the house, the decision was quickly made. And I have to spend the €5,000 anyway - whether on solar thermal energy, which according to your calculation saves me €56 per year, or insulation boards that reduce my heating costs by €4.45 per month.

So the decision is often not just about sense or nonsense, but which nonsense brings more benefit.

Amazing, so many facts in two sentences.

Since I don't like insulation boards on the house, the decision was quickly clear.

Just think about it.