You can use The Microsoft Windows Energy Savings Calculator to easily and quickly personalise your carbon savings and energy bills when you upgrade or virtualise.*
Its pretty simple, just enter in the number of servers, the consolidation rate or the number of servers after virtualisation to get get information about:
* The purpose of the calculator is to estimate server energy spend and savings. This calculator is provided for informational purposes only. Microsoft makes no warranties, express, implied or statutory as to the information obtained by using this calculator.