KMcK, indeed I did use the handy APS product selector and this is the machine that it recommended for "best performance." However, I am a bit nervous looking at stats that don't aren't listed in a scientific unit that is readily understandable to me.
Also, Park, how about one equation that begins with "W=" that would answer my question?
_*_*_*_*_*_*_*_*_*_*_*_*_*_*__*_*_*__*_*_*_*___*_* _*_*_*_*_*_*_*
So after further research, apparently the manufactures use VA because it's technically more accurate than Watts given this power factor thing.
In short, you can use W and VA interchangeably.
The caveats
(1) a Mac Pro is overbuilt; capable of generating 1000 Watts, but typically drawing far less, say 500W.
(2) The power supply is inefficient, so there is a "power factor" PF that says: if a supply is only 80% efficient you will need 625W going in to get your 500W. However, most modern power supplies are around 95% efficient, due to
http://bit.ly/oQVyb1 PF correction designs.
So, the caveats are small.
As for me, to be safe, I'm going to assume that it will be drawing 1000 Watts when the power goes out, and get the 1500 VA rated unit.