...what fonix232 said.
I just built a system. Gigabyte GA-H77N-WIFI mini-ITX, 8GB RAM, i3-3220T, three drives active and it measures 40W at idle from the plug.
40W is still quite high in idle, there was a guy who made it down to around 6W with an i5 3rd gen.
Unlikely that measurement was complete, from the wall, or with three drives attached - which use that much power alone. You could save a bit of power with SSDs - but you would lose storage. An atom-based motherboard would save a chunk of power at the cost of CPU strength - which little is needed for a pure storage medium.