While scientists all over the world strive to create a mechanical equivalent to the human brain, one guy is getting kind of concerned about these posthumans' power bills. They're pretty high! At least, at first.
Anders Sandberg has run a few back-of-the-napkin calculations on what it might take, power-wise, to sustain a theoretical mechanical brain:
How efficient could a postbiological civilization be? The current IBM roadrunner does 376 million calculations per watts. If we take my mid-range estimates of computing needs, 10^22 to 10^25 FLOPS, then a single emulation would need 10^13 to 10^16 watts. The total insolation of Earth is about 10^17 watts, so this won't do - there would be space for just a few minds on the entire planet. But current research on zettaflops computing suggest we can do much better. A DARPA exascale study suggests we can do 10^12 flops per watt, which means "just" a dozen Hoover dams per mind.
But this first projection is flawed, which Sandberg admits; it exists in a future that has borne a mechanical brain, but that is for some reason bound to the computing technologies of the early 21st century. Assuming advances like Quantum Dot computing, which Sandberg thinks could create a 200-2000-watt mind (apparently 2-20 times our current brain draw), and other, more distant/less grounded concepts, he sees a future in which a mechanical mind might be extremely efficient:
The quantum dot computer mentioned above is not the most effective computer imaginable. Using reversible computation there could in principle be calculations done at no energy dissipation. Unfortunately it would still be needed for error correction and interacting with the real world. A conservative bound would be assuming one irreversible operation every millisecond at every synapse, which leads to 10^17 operations and an energy dissipation of 3*10^-6 watts per degree - colder computers are more efficient. Using just liquid nitrogen (77 Kelvin) the energy requirements of a mind would be on the order of 0.0002 watts, 20% of an optical disc player laser. Even adding in the costs of cooling and manufacturing the hardware, it seems likely that this kind of postbiological human would be extremely resource efficient.