This could be fun and informative, just download this free program to calculate pi (windows based only).
ftp://pi.super-computing.org/windows/super_pi.zip
Then run the program to measure the time it takes to calculate pi to 2 million digits. Do it both while plugged in and under battery power and post your results along with the notebook model,CPU type & speed, amount of memory and hard drive size & RPM. If we can get 20-30 results(hopefully more) to be statistically significant I'll compile and post the results here in a week or so.