I have a pair of set-top box PCs I’ve been using as always-on servers. I used them because they’re silent, but lately I’ve been thinking about power consumption. They were pretty good when I bought them in 2006 and 2008, but there’s much better stuff available now. I spent £60 on a Raspberry Pi and some supporting bits; given that it uses roughly a tenth the power of one of the set-top boxes it will have paid for itself in about two months.
While reorganising everything I also decommissioned an old Netgear switch which was likely costing £100 a year to run. Maybe it’s time you looked in your networking cupboard too!