There's a lot of references to calculations made but no actual data in this article. The Prius Prime gets just over 4 miles per kwh on battery alone. The same car averages 54 mpg when in hybrid mode. The article references $3.40/gallon for gas. Average price of electricity in MA is 23.66c/kwh. So for the price of a gallon of gas, you can get 14.37kwh, or 57.48 miles, so still beating out gas (54 miles) even on an extremely efficient vehicle.