It's come to my attention that rumors are starting to fly about System z energy consumption. IBM has stated with pride that you can go green and save with the mainframe, and contrary to what you might be hearing, it is true.
There are a couple of easy places to go for real, hard data about mainframe power consumption. For example, see the IBM Get the Facts about the mainframe web page. It gives irrefutable evidence backing up the conclusion that "z10 mainframes can outperform HP Superdomes on power consumption by a factor of 2.5x or greater" when comparing systems with comparable capacity. One System z 10 Enterprise Class system a quarter of the total watts for power and cooling than four 64-Way HP Superdomes, which offer comparable capacity.
Also, check out the product pages on the web for the various mainframe systems, as follows:
Look under the Specifications tab, and you'll find the wattage for each system.