I don't understand these articles zero marginal cost in fuel? Take a chevy volt (I have one, so speaking from experience). it takes over 20kwh of electricity

I don't understand these articles zero marginal cost in fuel? Take a chevy volt (I have one, so speaking from experience). it takes over 20kwh of electricity to fully charge (there's at least 10% loss) and get a bit over 50 miles on charge. Lets equate this to a prius, that gets about 50mpg. the cheapest electricity rates in the bay area is about 11.5 cents kwh. or if it takes 20kwh to charge, one is looking at at least $2.30 to "Fill it up". That's not much cheaper than it costs to go 50 miles in a prius. If gasoline prices crater (as the article expects), it make gas cars even more affordable. Now arguably the electrical generation can be better for the environment than the car itself burning gas, but the argument on fuel costs seems very weak.