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.