Wake On LAN (WOL) is a feature where you can send a specially formatted network packet to a machine to wake it up when it is switched off. The on-board network ports of my virtualization cluster should support WOL, and the always-useful ArchLinux Wiki has a guide for how to enable WOL.
$ sudo ethtool p6p1 | grep Wake Supports Wake-on: pumbg Wake-on: g
By the way the BIOS option for WOL on these Gigabyte motherboards is implausibly called “Onboard LAN Boot ROM” which you have to set to Enabled.
It basically just works once the BIOS option is enabled.
$ sudo wol -i 192.168.0.193 74:d4:35:51:ab:86$ sudo wol 74:d4:35:51:ab:86 Waking up 74:d4:35:51:ab:86...
Edit: Don’t use the -i option. You want your WOL packets to be broadcast on your LAN.
Richard WM Jones 
I expect that might be where a bit of your power usage on non up machines is going. The power supply has to keep the systems down to a minimal “ready to start when pinged” level.
I’m not totally clear how these things work, but yes that must be correct. I assume the network card has a little ARM chip or something that keeps ticking over looking for the WOL packet.
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