Note that the power supply's rating is not indicative of the machine's usual consumption. When all else fails, you really should hook these things up to something like a Kill-A-Watt to measure it.

Yes, you're right, I assume the max draw is 15W, probably 5W at idle and perhaps an average of 10W?

Anyway it's much less than 175W-185W I'm using now with a Core i7 as a 24x7 server...