Today my next electricity bill came through, with a substantial underestimate on my actual power usage, and so I decided to do the maths. Turns out that Metis (the spare computer) sucks about 120W, and is responsible for over a third of my power usage. Ouch.
So I did some digging, and it turns out that sometimes Windows doesn't properly idle the processor for various reasons. Instead of executing a special "go to sleep until something happens" command, it just sits there doing the computer equivalent of jogging on the spot. There's many CPU Idle programs that purport to actually reduce your CPU temperature, but I've always been a bit sceptical of them.
Well, I downloaded a shareware one to see if it actually did anything, and just starting up the program was enough to drop the power consumption down to 70W. This is without even running the utilities "idle loop" - some hardware poking it does at startup is enough to make Windows start idling the system properly. That'll make my next bill nicer.