# how does human brain compare to a modern CPU in energy per bit?

Can someone compare the energy efficiency of human brain as a computer ? What is the energy in joules / flop ? may be some reasonable assumptions on the computational load of common tasks such as pattern recognition or speech synthesis can be used.

• Related (not a duplicate): physics.stackexchange.com/questions/2178/… – David Z Jun 28 '11 at 1:39
• The human brain doesn't use bits – Dale Jun 28 '11 at 5:48
• @David that was a very useful link ! – New Horizon Jun 28 '11 at 23:15
• This depends significantly on the task. Computers are much better at number crunching, but humans are much better at pattern recognition. (e.g. computers have trouble playing Go: en.wikipedia.org/wiki/Computer_Go) – blah Jul 28 '11 at 7:34

Human power consumption can be guesstimated as 100W, similar to the power consumption of an ordinary computer, plus or minus a few orders of magnitude depending on one's idea of "ordinary". A computer can do billions of flops per second, and it would take me many seconds or minutes to perform one with pen and paper, and furthermore I will make many more errors. If we assume that there is some other task which is stacked the opposite way, i.e. a human can perform it a billion times faster than a computer, and that both of these are in some sense extreme cases, then given some more "fair" test we can say that ratio of the efficiency is probably somewhere between $10^{-9}$ and $10^9$.
I'm not a big fan of Ray Kurzweil, but he does have a good summary of some relevant data in his 2005 book The Singularity is Near. There's a lot of ambiguity in trying to estimate the number of fundamental operations involved in a certain neurological process. Kurzweil refers to "synaptic transactions," and equates one of those to be something like $10^3$ "arithmetic" operations, where I imagine that he means something roughly similar to my definition of an arithmetic operation above. Anyway, subject to all these ambiguities, the studies he cites estimates of $10^{14}$-$10^{19}$ Hz for the rate of operations per second. If the brain draws ~10 W (Dan's link says 20), then this is an energy consumption of $10^{-13}$-$10^{-18}$ joules per operation. Since a desktop computer currently does $\sim10^9$ arithmetic operations per second, this makes the brain more efficient, as measured by joules per operation, by about a factor of $10^6$-$10^{10}$.