We're understand that the Hadoop is a low-cost way to manage and process the massive data since it has been designed to run on a lot of cheap commodity computers. But, the electric power costs also should be considered when evaluating cost effectiveness. Have you thought them? Since It's a fault tolerant system with active replication, a few servers could go anytime into power saving mode without data loss.
I heard that some guys are trying to handle this problem. See also : On the Energy (In)efficiency of Hadoop Clusters