Power saving
From MythTV Official Wiki
Automatic powerdown/wakeup
See http://mythtv.org/docs/mythtv-HOWTO-11.html#ss11.5
CPU
Powernowd can lower the clock frequency when the CPU is idle
Display
Use a display adapter without fan. Those with fan usually have higher power consumption than those without.
DPMS will turn off your display when the PC has been idle for a while