Hi Martin,
thank you very much. That was just the right hint for me to spot my mistake .
As I mentioned before, I had to combine weather data from multiple weather stations in relative close proximity. I worked on a relative complex python script, to concatenate and convert everything into one file readable by PV*SOL. My mistake was that I haven't payed attention to the format of the different timestamps. While some data was saved under a system time, other data was saved in the UTC format. In summertime I got an offset of +2 hours on days, where I had to calculate the average with the UTC data. I quickly aligned my timestamps and voilà ... everything is working fine.
I guess the core message of my question is:
Folks, keep an eye on your timestamps while merging different data sets .
Again, thank you very much. Kind regards,
Niklas