@lminiero So, in 2019 I published this: https://www.forbes.com/sites/jasonevangelho/2019/05/06/benchmarked-the-most-power-efficient-ubuntu-19-04-flavor-will-surprise-you/
The takeaway was that GNOME had a very similar power envelope compared to Xfce (surprising, yes) and KDE was indeed pretty power hungry.
MATE was the most power efficient at the time. I am VERY much looking forward to revisiting these tests with better tools and a few years of optimizations in the rearview mirror.