Andrew Jones at ZDnet looks at power requirements as one of the key challenges for Exascale:
There are a range of estimates for the likely power consumption of the first exaflops supercomputers, which are expected at some point between 2018 and 2020. But probably the most accepted estimate is 120MW, as set out in the Darpa Exascale Study edited by Peter Kogge. At this figure, the supercomputing community panics and says it is far too much — we must get it down to between 20MW and 60MW, depending who you ask — and we worry even that is too much. But is it?
This comes down to dollars and cents. One Megawatt will cost you roughly $1 Million USD per year at today’s rates, and an Exascale system could cost more to operate for twelve months than it takes to purchase the thing.