http://www.faqfarm.com/Home/Electrical/1987 is a quick explaination of resistive vs reactive loads. Many electric utility companies provide substantial discounts (or penalties) to commercial and industrial power customers based on power factor observations and commitments.
It's ususally a little easier to just ballpark it. 1U p3 or p4 is 1 amp, 2U or dual xeon 1 3/4 to 2 amps. If you have dual power supplies depending on the design you can use a single value or may need to double the estimate.
You should leave at least 20% extra so if you are on a 20 amp service you need to have at least 4 spare amps of head room. you also need to plan for switches, firewalls etc.
Atlanta, GA and Las Vegas, NV. Colocation