Linux Journal builds a low power NAS with an Arm-based server and USB hard drives.
As an experiment, and finally to get rid of that large, inefficient and ugly tower case, I decided to use the new Trim-Slice as the base for an ultra-low-power, ultra-small replacement file server. The Trim-Slice is built on the NVIDIA Tegra 2 platform, and the specific model I purchased features a 1GHz dual-core ARM Cortex A9 processor, 1GB of RAM and a 32GB SATA SSD.
via Linux Journal.
The ultra low powered would be xeon e3-1220LV3. That baby has power to run virtualization and then some. It uses only 13w. The v2 is 17W. Most folk like the higher power versions because they can keep up with modern processors. If you go for xeon e3 higher wattage check out disabling cores to reduce the power draw. Always check the specs to see what you are getting.
It really makes a difference when go for low power but performance that usually means a mis-match.
The motherboard I like to stick with is Supermicro but there are many good alternatives.
Again it is expensive around US$500. I like IPMI and other remote configuration tools.
If you can go for a second hand IRU rack unit you can stick it in the garage/basement and out of the way.
It is a shame that basements are not that common now days.