"Doing P-to-Vs, or physical to virtual conversions, there's a black art to it if you will," Pradel says. The conversions "don't happen in a vacuum. That happens in conjunction with different business units' schedules and the ability to withstand downtime."

3. Virtualization automatically reduces power use

If you have consolidated onto fewer servers, it might be tempting to say "I've solved my " Not so fast. While you now have fewer servers using up watts, each server is running at a higher CPU capacity and has greater power needs. At in Massachusetts, a virtualization project has actually increased overall power use, reports network and systems director John Turner. Although Brandeis dramatically reduced its number of servers, it is now offering more services to users because spinning up new VMs is so easy. Each new workload increases power use.

"If you walk behind the racks of virtualized servers, the heat is just pouring out of those guys," Turner says. "We're seeing heat dump into these rooms like never before."

Another issue to consider: If you're shutting off lots of servers, a data center has to be to prevent cooling from being directed to empty space, explains APC CTO Jim Simonelli.