Vehicle-to-grid (V2G) describes a system in which plug-in electric vehicles, such as electric cars (BEV), plug-in hybrids (PHEV) or hydrogen Fuel Cell Electric Vehicles (FCEV), communicate with the power grid to sell demand response services by either returning electricity to the grid or by throttling their charging rate. Vehicle-to-grid can be used with gridable vehicles, that is, plug-in electric vehicles (BEV and PHEV), with grid capacity. Since at any given time 95 percent of cars are parked, the batteries in electric vehicles could be used to let electricity flow from the car to the electric distribution network and back.
A great deal of research into V2G is begin done at the Delft University of Technology, see also: https://en.wikipedia.org/wiki/Vehicle-to-grid