How to turn power on remotely.
This is achieved with a wonderful technology called "Wake on LAN (WOL)". Here is how it works: you turn the power off completely with the power switch while not unplugging the PC from the socket. You may notice that there is a little light on your network card (if there is not, the card is either unable or unconfigured to do this trick). Then you send a magic packet to MAC address which is a 6 byte unique address on the network card. Upon the receiving it the card wakes the whole computing mammoth of yours. What to look for:

• BIOS settings. In our case it was set not to do WOL.
• Get some program to send the magic packet.
• Find out the MAC address on all nodes. This can be done by running ifconfig on each node as root. Build your database and keep it in a safe. Anybody (?) can wake your nodes if they know the address (and presumably live somewhere on your local network).

How to save power (on desktops)
I have no idea! If somebody knows, please tell me.

How to turn power off remotely.
This is achieved with the standard "poweroff" command, that Linux does not like to let users run freely. This means you have to log in at each of 24 nodes as "joe user" (since rsh by default ignores root logins), su and then do poweroff. If you do it once you will know how it feels. So I tweacked the /etc/pam.d/poweroff file by adding "auth sufficient permit.so" line allowing all users to do poweroff. Then the job became a trifle: run a script that would rsh to each node and run poweroff (having set ~/.rhosts to save the pain of having to give passwords; the script is here).

raison d'être for all this:
2002 year electricity bill: 16.5 mln yen (about $140K), for a lab of 55 members.