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Yes.

Someone did benchmarks of an overclocked 27" iMac a while back. He was still running under 200W while under load. Your total load under your most demanding conditions will be under 250W. Normally you should expect to be around 140-180W, depending on whether or not the external disks are asleep or spinning away.
 
Definitely a good UPS. I have the 850VA model, run an iMac i5 from 2010 with modem and Time Capsule and I'm almost always at quarter load.
 
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