Gas heating without solar thermal?

Hello everyone!

A quick question:
Why do people actually add solar thermal systems to their gas heating?

Because they have to due to the Energy Saving Ordinance?

Wrong, you are also allowed without solar thermal systems if other measures cause the Energy Saving Ordinance to be undercut by 15%.

Example:
I am planning a KfW-55 house with gas heating, solar thermal system, controlled residential ventilation with heat recovery, and super insulation.

Now I leave out the solar thermal system. What happens?

I lose the KfW-55 status, am only KfW-70, so 2,500 € in subsidies are lost. In return, I save 6,000 € because of the omitted solar thermal system. The consumption costs due to the omitted solar thermal system increase by 50 € per year, that makes 1,000 € over 20 years. So I still save 2,500 € in total.

Savings:
6,000 € saving from omitted solar thermal system*
-2,500 € loss of KfW-55 premium
-1,000 € higher operating costs due to omitted solar thermal system*
=======
2,500 €

Sounds like an attractive alternative, or is there a major miscalculation somewhere?
The whole calculation would also work with a KfW-85 house, which without the solar thermal system would still qualify as a KfW-100 house.

Best regards

jx7

PS:
Please do not bring up heat pumps or pellets, this is not meant to spark a fundamental discussion about gas or heat pumps (with drastic increases in energy prices, a heat pump eventually becomes more economical), the question is initially only "If gas, then possibly without solar thermal?".

* Source: xxx-Heizungsvergleich-de "Gas + 5qm Solarthermie"

Does your comment generally refer to the use of solar thermal energy to support heat generation, or is it specifically related to the example?

Of course, the use of solar thermal heating and hot water preparation depends on many factors in the individual case, but the fact that these can contribute more or less significantly to coverage should not be fundamentally excluded.