The processors themselves all seem to be pretty efficient at idle (there's apparently no real reason to get a T processor. It just caps performance). It mostly depends on everything else. The only place I know of to get this kind of info is this thread[0] where people have been building this spreadsheet[1]. Some threads on servethehome.com also report when some pcie cards have broken power management which can destroy idle efficiency.

[0] https://www.hardwareluxx.de/community/threads/die-sparsamste...

[1] https://docs.google.com/spreadsheets/u/0/d/1LHvT2fRp7I6Hf18L...