ACPI Shutdown & Wakeup
If your motherboard is capable, it is possible for MythTV to shut itself down when it is idle and wake itself up if there is a recording scheduled. One method is by using the built-in ACPI Shutdown/Wakeup via RTC alarm function of many motherboards. To see if this might be possible with your system and a guide on how you might get this working for you, visit: ACPI Shutdown Wakeup