The most ecological solution generally is sharing one dedicated server
between many users. Each of them can administrate her own virtual machine. By
sharing, the load is more constant: the hardware does not spend as much
electricity being idle. The hardware in question usually is server-specific
too (no video card consuming energy for nothing).
You should also take into consideration that most people have a WiFi router
at home, which is idle most of time too. Instead of that, it would be better
to use a single board computer as a server and a WiFi router in one.
Also, for privacy sake it's much better to the computer at your home instead.