I guess some people around the world have quite cheap utility bills! For me, it's either a Raspberry Pi type of power consumption, or else a server that only powers on when needed. But I haven't learned yet how to do the latter, if possible at all.
In fact this is a nice place to ask: how would you build a "something" that monitors the network for packets sent to powered-off machines, then somehow caches the request, powers the destination machine On, and finally lets the request continue to its target? Has this been tackled anywhere? There must be tons of people wanting a homeserver but living in places where electricity has a considerable cost...
The best days of my life are the ones where I don't see or speak to another human being. Blessed days.