The design decreases the efficiency of the car in total, through increase in weight, increase in mass and probably a few other things we haven't thought of. It makes up for it by allowing you to replace gasoline with electricity, which is much cheaper.

Less efficient, much cheaper, and hopefully less environmentally harmful.

I like what this solution offers - a chance for someone like me who will probably never buy a new car to easily modify an older car to run at least partially off electricity.