[simonhn]

I also found the following article invaluable when building my ATMEGA328-based project using watchdog timers and a battery.

http://donalmorrissey.blogspot.de/2010/04/sleeping-arduino-p...

There is some cumulative error (maybe one second per hour) after sleeping in increments of 8 seconds (the longest interval possible with the watchdog timer) because of the time lost starting up and shutting down, so this approach is not 100% accurate.

Running direct from 3xAA batteries (=4.5V) means that you can avoid a voltage regulator from 9V too.