KFW55 is standard today... The savings in running costs from 55 to 40 will certainly not be 500€ per year...
So it simply doesn't pay off no matter how often I try...
Edit: does anyone perhaps have a comparison value here?
Do you want theoretical or practical values?
The savings at a gas price of 10 cents should be at most 150€ per year (practical, standard single-family house kfw40 vs kfw55)
What is worthwhile is photovoltaics (as long as there are still cheap modules, they have already become 25% more expensive in the last 6 months)
With electricity prices from 50-60 cents, a battery storage system also becomes interesting :D
And I am afraid that we will have electricity prices in this range in the near future.
I am already paying 37 cents now, from 43 cents my storage pays off.