Unlike the TOP500 List, which lists 500 of the worlds fastest supercomputers, the Green500 List lists the most energy-efficient supercomputers that made the TOP500 List. So for the Green500, it’s not about how fast, but how energy-efficient. That’s why China’s Tianhe-2, which topped the November 2013 TOP500 List, did not even make the top 10 of the Green500 List for the same month. Rather, it ranked number 41.