If you get an atom line CPU a smaller I/O suite. Say a dell 3040 or 5070 and your at 4W idle - similar to the Rock5 (and cheaper if you're buying used on ebay). When you get to the full fledged core lines you're looking at 8-10W as a baseline. My Pi4, when I was running one, was 2-4 Watt @ idle.
I don't have it on a kill-a-watt even though I should. As the person below you said, it's like 1.5 to 2x power consumption vs a pi4 at idle (Under 10w), but that is still comically small amounts, about the same as a Google Nest Hub.
If you really want to deep dive on this topic see parky towers https://www.parkytowers.me.uk/thin/