PowerTOP

PowerTOP is a Linux utility that can monitor and display a system's electrical power usage. It is useful as a hardware monitoring and diagnostic tool. It is among the most powerful battery stretching utilities for notebook computers.

Kernel
Both  and   must be enabled in the kernel for PowerTOP to work properly. Enabling  might be a good idea as well.

If the above options have not properly been enabled Portage should display warning messages at the end of the emerge.

Emerge
After setting USE flags, emerge PowerTOP:

Configuration
(Software configuration.)