Whoops, 16 kWH is the capacity of the battery, but they only charge up to 80% and discharge down to 30% capacities to extend battery life.
http://www.caranddriver.com/news/2011-chevrolet-volt-first-drive-car-news
So make that question, what do you pay for 8 kWH?
get your electric bill out and read it.
Well! If you look at your electricity bill the first few KW are real cheap! When you get into the 4th or 5th tier pricing, you will go broke! It is usually about triple the 1st tier amount. So you will be screwed no matter what.