My guess is, that those 5-6W on idle that gets thrown around in so many blog posts is probably just the CPU in idle state and not the whole board with io and other stuff.
Well, no.
My Futro has some old thin client AMD CPU and the HP Elitedesk g3 mini has an i5 6500T. Both idle at 5-7W (the HP is slightly higher).
My optiplex is just an SFF, so still a regular CPU (i5 6500, without the T), regular SFF power supply, etc. And that one draws 10-15W.