The 5th annual list of the most energy efficient supercomputers was published recently. IBM took the top four spots and 18 out of the top 20. The Green500 list is dominated by BladeCenter clusters and the Power-based Blue Gene supercomputer.
The list is compiled and sorted by megaflops per watt. The winner was a BladeCenter QS22 Cluster out of the University of Warsaw in Poland running at about 536 megaflops per watt. Incidentally, the same cluster is ranked 422nd in the TOP 500 supercomputer list.
Perhaps most impressive of all the supercomputers is a BladeCenter QS22/LS21 cluster run by the U.S. Department of Energy. It is #1 on the TOP 500 list and #4 on the Green500 list.