How much power, in watts, does the brain use? When  IBM's Watson won on Jeopardy a few years ago, it did so using a room full of servers with a cooling system and a fat power feed, competing against a couple of humans powered by the equivalent of a tuna fish sandwich every several hours.  How much power does the human brain actually use?

Notes: If other assumptions are necessary, just state what you're answering for (e.g. healthy adults). If you know of differences between groups (e.g. people who are awake vs. asleep), please mention both.   

References to scientific papers, especially reviews, are helpful. 

This paper, which might be the one discussed here, discusses power usage of individual neurons but not the whole brain.
20 Watts.  


Watson mentioned this in 2011: "The human brain only requires 20
watts of power to operate ... Watson? About 20,000 watts."  An interview with John Kelly (an IBM senior vice president and director of IBM Research) also puts the brain at "about 20 watts" (but puts the machine at 85 kW; the inconsistency doesn't matter for this question).
Popular Science also says 20 watts in an article explaining how simulating it in 2009 technology would take tens of megawatts.
Glenn Elert's high school students cite a collection of resources concluding 20W, as 20% of a 100W full-body power consumption.  That's the source cited on Wikipedia.
The 20W figure is cited like common knowledge from "several different sources" here, but those sources aren't named.