Problem:  

Tool that coverts electric power from watts (W) to current in amps (A).  How to choose the correct UPS based on server load.  Ensure backup UPS unit has the correct voltage and amps for runtime.

Solution:  

W = V x A.  Use the voltage from the UPS batteries -not 120V!

Use the Watts-to-Amps tool to calculate Amps/runtime based on server load/Watts:

APC Network Administration UPS