Let's start with Shiunbird's setup:
Shiunbird wrote:My whole setup draws 135W (2 x86 systems, 8 hard drives, a switch, a modem, 2 ARM boxes). If I would go non-x86 (use my POWER5 as a server, for example), I'd go to 500W+. The POWER5 eats 35W on standby!
If you are aware of low power, non-x86, relatively modern stuff, I'd love to hear about.
I recently purchased an Odroid HC1. This is practically an XU4 without USB 3.0 hub but built-in USB3.x-to-SATA bridge supporting a single 2.5" drive. It's also passively cooled, but can get somewhat hot when all eight cores are in use permanently and fully clocked up. Works well with Armbian and can fully exploit the 1 Gbps NIC attached via USB 3.0. With a Samsung SSD 850 EVO I've seen more than 350 MB/s R/W running IOzone, so more than enough to saturate the network connection. Odroid also sells a matching case/hood and in the near future they will offer the HC2 which will support a single 3.5" drive with matching case. Have a look at this thread in the Armbian forum for more info about the HC1 and HC2. So far I'm pretty happy with the HC1, it will provide my machines with NFS and TFTP services in the future and replace an older dual-core Atom machine.
Another approach would be to power machines only if needed, e.g. via a PDU. This is how I keep my power bills low. I'm running a Raspberry Pi 1B which controls the PDUs and the remote control facilities of my machines (if available), so power is only used if need be (apart from the power needed for the Raspi, the PDU and network switch and possibly a console server).