The 1000 Watt PSU in the desktop is a sign you may want the larger unit. This is only to let you have time to do an orderly shutdown. Not to keep gaming or printing.
Most advise 2X your biggest total load. We need a P3 Watt Meter (see google) to really know.
Bob
I need to purchase a new UPS battery back up and would like some advice as to how much wattage I need. I am looking at one that has 1080w for $119 and another one that has 1500w for $179. My PSU is an XFS 1000w, I will have at least the following peripherals attached - Computer, LCD monitor, 2 printers, several external hard drives, and 1 or 2 powered USB hubs.

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