Are you supposed to place a Kill A Watt meter between the PSU and UPS like so:
PSU--Kill A Watt--UPS--Wall Outlet?
or are you supposed to place a Kill A Watt meter after the UPS like so:
PSU--UPS--Kill a Watt--Wall Outlet?
It depends on what you want to measure with the meter...
A note about measuring power consumption (or anything, for that matter) -- the longer duration you can measure for (assuming your meter generates an overall average since it was connected), the more accurate your results can be.
You may also find it interesting to measure separately after the UPS has been un-plugged for a while (to simulate a power outage) as well to learn how much power is consumed by the UPS charging the battery.