Hatter makes some great points here. Here's what I've done now with my older, 3rd-party retail outlet-sold APC Back-UPS XS 1500 (no additional battery hooked up, just the original it came with) :
1. Loaded PowerChute Personal Edition for WinXP on my VMWare Fusion XP vm slice, making sure of course to enable the USB port in the VM that the UPS is plugged into (I recommend direct USB connection, never thru a USB hub for a UPS).
2. PowerChute software is up and running, I ran the self-test to makes sure all is well (I've had the UPS for about 3 years, even though I've never loaded any software to monitor it). Mine is functioning properly.
3. PowerChute is really nice software IMO. It is showing that I am currently using 449 Watts of power! That is quite a lot, but considering what I'm running, not that bad actually. It is still only half of what this unit can do overall, though I would never want to tempt the upper limits.
4. Also, as I run more app's, taxing the RAID-enabled drives more, using more memory, etc., the Wattage increases. You can see this dynamically in the PowerChute Current Status area. As I've opened several VM slices (WinXP, Win2K3 Server, Ubuntu Linux, etc.) the wattage is increasing to a range of about 450-490 watts. My estimated Battery Time, given my current load, is 12 minutes. This is in line with what I've seen in a real world scenario, having lost power about a month ago for several hours.
Hope some of this detail is helpful to someone.
Cheers!