When ordering colocation services, you need to know the size of circle that you need by figuring out how many Amps your system will draw. Here is a quick example on how this power calculator will help you to find that out.
First of, you have to calculate the total maximum power draw in Watts, then use the power calculator to convert it into Kilo Watts (kW), and after that you can use the number of kW to find out how many Amps your system will draw.
Let say you need to fill up a rack with the following:
1 router: Cisco 7200 VXR Series Router has a maximum power draw of 370 Watts
1 switch: Cisco Catalyst 3560-24TS has a maximum power draw of 74 Watts
30 servers: HP ProLiant Servers with 80 Watts each
Total Watts = 370 74 (30 * 80) = 2844 W
2844W = 2.844 kW