Adding my data as well:

My server is diy desktop pc - mbo MSI Z270-A PRO with celeron G3930 and 16GB RAM, 3x SSD on 550W PSU, idles at 23W. After adding another 3.5" HDD consuption went up to 34W. 34W in Ctoatia is around 34€ a year.

Some SFF PCs are at 10-15W. SBCs like rpi should be below 10 W, but dont think you can get anything new for 80€