Chump change.
A computer hibernating uses almost no power. Running your central A/C for 15 minutes a day uses more energy than a computer sleeping for 18 hours and in use for 6 hours a day.
A/C, heating, and clothes dryers by far make up most residential energy usage. There's lots of places you can see wattage usage and see how much it costs in your state, like http://michaelbluejay.com/electricity/howmuch.html