In order to properly size a UPS to a multi-slotted switch, such as a Cisco 4506:
1. Do the switch's power supplies, modules and fan unit of the switch draw power from the UPS?
2. Do the modules and fan unit draw power only from the switch's power supplies and therefore a switch's power supply is selected based on the modules and fans installed...and then the UPS is seleted based on the switch's power supply?
Option 2, just make sure the power supplies you purchase provide enough power for all the modules you want install.