Here’s a link to a really useful pdf document from HP which provides considerations when sizing the power requirements for a single, or rack of servers.
It’s explains clearly the meaning of the following terms that you will come across: Input Line Voltage, Device VA Rating, Device Input Power, BTU’s required for cooling, Input/inrush and leakage current.
You may want to also check out the following HP links:
HP Power Advisor – Use this for HP Proliant G6 servers
HP Power Calculator – Use this is for HP Proliant G5 servers and below.