I would say this idea of V2G would be good if we could honestly say that the energy storage of the car is a good one and the way we got the energy into the car is also a good one.  However, isn't this the very thing we have been struggling with for many years?  A lightweight, high capacity storage device that fits in a car with efficient conversion?  Typically a charging a battery consumes about 150% of the energy it is capable of giving back to you.  About a year I attended a presentation in Boulder Colorado by Dr. Dean Patterson (IEEE Fellow, Lincoln Ne.) where he talked about this very thing.  As it turns out, today's automotive battery technology still has a long way to go.

If you don't trace the true source of the energy you're using, you're fooling yourself as to it's ultimate cost effectiveness.  I propose we borrow from the "five whys" technique, but slightly modified. 

For every joule of energy stored in a device (e.g. the car battery), ask yourself three questions:

 - Where did it come from? 

 - What did it cost to get it? 

 - What does it cost to store it?

When you have answered these, add up the total cost.  Now, if the source of energy is not from something primary like the sun or the wind, ask the three questions again and add the total cost.  After five iterations of this method, you begin to see what this little bit of energy you're trying to put back on the grid is really costing you. 

-Doug

we are in the process of reducing the use of carbon parts based fuels in any way possible... we can't change the past, isn'it it? but we can change the future ahead! hydrogen, electric and hybrid vehicles are the viable alternatives. Solutions already presented. Hybrids has a bigger economic value.