The beauty of electric plugin vehicles is that they would also serve as a huge national distributed battery.

All connected to the grid when parked, 100s of millions of cars would provide energy storage to buffer inconstant wind and solar power inputs. 

And when your local power went out from a storm (it happens 4 or 5 times per year here) threatening to freeze your home in winter, the  huge capacity (80 kwh or more, enough to power your home for days) in your electric car's battery has you covered until things are repaired.

When 50% of the US does drive plugin electric cars, that will amount to days worth of energy storage across the country, allowing renewable energy and electric power demand fluctuations to be adjusted with backup fossil and hydroelectric sources.

Even the American Wind energy Asociation, an advocate for wind power, admits the supply interuption problems with going over 20% renewable power use on the grid.  These car batteries would push that theoretical limit way up!

Maybe only 20% of power will come from fossil fuel someday?  That would be very good for mother earth.