Available average temperature data

Nowadays, temperature data is becoming easy to get hold of. There are many locations on the web holding data, covering many decades. The best place I have come upon is listed below together with their "commercial":

THESE DATA CAN BE USED FREELY PROVIDED THAT THE FOLLOWING SOURCE IS ACKNOWLEDGED:

Klein Tank, A.M.G. and Coauthors, 2002. Daily dataset of 20th-century surface
air temperature and precipitation series for the European Climate Assessment.
Int. J. of Climatol., 22, 1441-1453.
Data and metadata are available at "https://eca.knmi.nl".
However, it seems the site is no longer available, but this site, "https://www.ecad.eu/dailydata/datadictionary.php", seems to carry something similar

I tend to use average temperature data from Copenhagen (Landbohojskolen), Denmark (close to Malmoe). There are a lot of data available (1874 to 1999). To get slightly more recent data, I also use data from Hammer Odde, which is only a small bit further northwest in Denmark. (1984 to 2005).

For those living in other places off Sweden, it can, for example, be pointed out that data also exist, for example, for Stockholm.

For those wanting to experiment, but not willing to try to fetch the above data themselves, I have created a webpage allowing you to make calculations on electric heating cost and comparing different heat pumps to see how heating cost can be reduced.