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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.

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  • $\begingroup$ Brain energy consumption is not estimated correctly. It takes all the mitochondria in the entire body to operate the brain. When you say a computer uses 500 watts you are referring to the power supply not just the power running into the cpu and memory. Ergo your entire body is there to power your brain. So so however much energy you are burning in your entirety is the energy required to run your brain. You can extend this to external sources and tools as well, fires, stoves, cars, airplanes, spaceships. They are as mych a part of us as we are of them. $\endgroup$
    – user3517445
    Commented Mar 23, 2022 at 2:17
  • $\begingroup$ @user3517445 our large motor muscles expend a lot of energy doing work moving things around in our environment which I wouldn't count as part of this answer. In your computer analogy, this question is asking about power consumption of the computing units, not the whole power supply incl. power to the monitor, speakers, etc. $\endgroup$
    – WBT
    Commented Mar 30, 2022 at 17:39
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    $\begingroup$ The idea that "it takes all the mitochondria in the entire body to operate the brain" is silly. (1) Since the brain of a person whose limbs have been removed is as functional as it was while those limbs were attached, it must be false to say that it "takes [the mitochondria in those limbs] to operate the brain." (2) It is perfectly sensible to ask how much energy an individual organ consumes. (3) The author's claim becomes even sillier as it is extended to external, supportive energy consumption (cars etc). By this logic, it takes the whole power output of the Sun to run a brain. Not helpful. $\endgroup$
    – Larry Gilman
    Commented Jun 27 at 12:55

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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.
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