When on: 90W
Booting up/resuming from hibernate: 120W
Suspend mode: 1.2W
I was surprised to discover there was still 0.5w going through when entirely off, but such is life.
So, the difference between Off and Suspend is 0.7w. The time to resume from Hibernate is about a minute - so that's 120Wminutes used in powering on. This is equivalent to the power used in four hours of being in suspend mode.
Therefore, if I'm going to be using the desktop within the next four hours then suspend mode is more efficient than turning it off or hibernating it. And leaving it in suspend for a whole day uses additional power equal to 8 minutes of being on normally. Or what happens all the time if I wander off to the toilet, or to make a cup of tea.
Speaking of which - making a cup of tea uses 2700W for about a minute. Or, in other words, making a cup of tea is equivalent to keeping the desktop suspended for three days.
As it is, I value the utility of instant-on computing highly (being able to turn the computer on within 2 seconds, use it, and then suspend it again), and so am happy to expend a tiny amount of energy for that. If it was costing me 60w to keep the desktop suspended then I'd definitely be turning it off when not directly using it.
Original post on Dreamwidth - there are comments there.