Powerdevil is a new power saving daemon for KDE4. It's an utility for Laptop Powermanagement and it's part of kdebase
from 4.2 on. With Powerdevil you can set up preferences for lid Closing, Brightness, CPU scaling, Idle time, both when connected to AC and when on Battery.
Open Systemsettings/Advanced user settings/Power management:
Left click on the Battery Monitor Settings: