Anders Sandberg tries to calculate the amount of energy it would take to sustain an uploaded human consciousness
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. Quantum dot cellular automata could give 10^19 flops per watt, putting the energy needs at 200-2000 watts.
That is between 2 and 20 times the current wattage of a current human. However, we bio-humans get our energy through the inefficient method of having plants collect sunshine (at about 3%) efficiency, then we either harvest them and eat a small part of them (expending a lot of agricultural energy) or have animals eat them (at a few percent efficiency) and finally we eat the result, again with a few percent efficiency. A brain emulation of this type would just need a few square meters of solar panels (plus night-time energy storage). In terms of area and energy required, these postbiological humans would have far smaller material requirements than we do. They could also run slower to save energy.
[via Charlie Stross]