[walrus01]

You can if you want an absolute maximum of what the thing can draw as a load, it'll be labeled, but very often the real world use of a thing with an AC to DC power supply is a very different wattage figure from what the power supply is theoretically capable of.

Such as having an ATX midtower size 'gaming' desktop PC with an 850W power supply, that might be measured at 350W at the wall under full CPU+GPU benchmark load.

[sgtnoodle]

Right, so if your max rated load is below your UPS's capability, it should work with plenty of margin and you can stop worrying about it. If your max rated load is higher than your UPS's capability, then you can either analyze the actual load more to see if it still works, or just buy a beefier UPS.