I am using an arduino pro mini 328. I would like to be able to read a sensor data and then power down. After an hour or so, i would like for the arduino to wake up and take another reading. Can someone help me getting started on this.
I'm always using this page as a reference when working with low power applications 
I think I'm mainly worried about going into power down mode. I need to put the arduino to sleep and wake it up without me physically being there. The arduino is supposed to be collecting data every hour.
ahbba:
I think I'm mainly worried about going into power down mode. I need to put the arduino to sleep and wake it up without me physically being there. The arduino is supposed to be collecting data every hour.
The page hansibull linked you to has a description of how to do exactly that! It's in the section "waking from sleep with a timer".
Yeah it is a good and helpful page, but i need to have the arduino power down for an hour. It only explains how to wake it using a watchdog timer which has a max wake up time of 8s, meaning it will wake it up every 8s.
Your Arduino is quite capable of counting to 450.
Could you use a RTC with alarm, use that to trigger an interrupt once an hour to wake up the Arduino?