Futurist Ray Kurzweil predicts the Technological Singularity, that point around 2030 when a computer has the brainpower of a human. The next step would be for humans and computers to evolve together into hybrid, silicon-based lifeforms. Now deep thinkers are considering how a post-biological civilization might work, and how much power it might take to sustain it. Would it be greener than today's meat puppet planet? According to Anders Sansburg:
"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."
So, today's computers can't cut it. But what about computers 30-50 years from now? Smarter than everyone in the world combined, they'll be able to think up even more efficient ways to compute, designing solar cells and batteries to keep silicon humans alive indefinitely. Or maybe they'll start wondering why humans need to be involved at all. Silicon humans? Sounds like an oxymoron.