The dual core atom from the 2010 era is a strange beast. True it idles at less than 4 watts, but the chipset they were paired with draw 15 watts or so.
My estimates for my power usage are:
Chipset/motherboard: 15 watts
CPU: 6 watts
2 hard drives: 10 watts
really old crap PSU from 2002: 20 watts
Although I didn't measure each component by it self, all the power requirements and specs are posted online mostly. Except my really old PSU, I can only assume that is where the power is going. I probably should have bought a new PSU for the build, instead I bought $20 in adapters just to make the PSU work with the board :) But my killawatt meter clearly shows the box drawing 50-55 watts around the clock.
And if you are nosy, this is what the dual core atom box runs: http://shorewall.net/XenMyWay.html
I got tired of having a separate web dev box and firewall box in my house. So I stood up Xen on a debian OS on the atom hardware, and now my firewall and web dev box are Xen guest. Make for fun Dom0 host upgrades when the internet is being provided by a guest that it is itself hosting...