power comsumption low and raise then

I am running kernel 2.6.24.1 on my Core2Duo Laptop. I trying to measure and lower the power consumption.

I noticed that after I logged in and started KDE (3.5.8) the system needs only little power (about 9 W, measured with an external power meter, no battery plugged in). But then usually after 10-15 minutes the power consumption raises to 22-25 W. I did not start any extra programs, I do not touch the system at all just watching the powermeter ... The system is idle all the time and I do not see any significant output in syslog.

I trying to find out what might cause this behavior. But I am at a loss. Can anyone help me to find out what is causing this behavior?

Steffen