Robert L. Mitchell, writing in ComputerWorld asks, "Not so green computing: Is Windows an energy hog?" A Sun customer, Mallory Forbes, senior vice president and manager of mainframe technology at Regions Financial Corp. in Birmingham, AL, says, "The utilization rates for our Windows boxes are dramatically lower than for our Unix boxes".
UNIX boxes tend to be more expensive than Windows boxes. They are also better at multi-tasking, so it is not surprising that people try to get more out of their UNIX boxes (because of the cost) and that they are able to do so (because of the legacy of time-sharing). It is not unusual to find Windows shops with a one-box, one-application strategy.
With Virtualization technique like LDoms and Solaris Containers, UNIX managers should be able to keep those utilizations rate high. Windows users can now use VMware (and soon xVM) to consolidate lots of Windows energy hogs into nice efficient, powerful servers like the Sun Blade 6000 or the new Sun Fire X4150.