In the meantime, I have collected enough consumption data for gas consumption (mostly heating, some little bits for hot water), and at various temperatures outdoors, including, very cold temperatures.
As it turns out, the gas consumption correlates well with the outside temperature (daily average temperature in 2 m height, as officially recorded close to my place by the weather authorities).
With datasets of daily temperatures for 2019 and 2020, we can then estimate the total gas consumption of these years.
The 2019 predicted consumption:
The 2020 predicted consumption:
For German standards, everything less than 100 kWh per m2 and year is quite reasonable. I need to admit that not all of the rooms are fully heated all the time, but well, it is not all that bad, comfortable, and affordable.
Here an energy rating chart – in German – around 100 is a good value, above 200 … there it is getting really bad, wasteful and expensive!