I came across this article today. http://http://www.dailymail.co.uk/news/article-1193070/Shoppers-cars-soon-able-power-supermarkets.html" [Broken] Does the energy they get from the cars come from the car losing gravitational potential energy, in which case the cars will have to expend more energy to get out of the dip (unless it is going downhill) but the energy from the engine does not go directly into producing power for this mechanism, or the cars' kinetic energy from the engines putting the energy in, in which case it cannot be 'green' energy. I think it's the first one. but I showed it to someone, and they think that the enrgy comes from the kinetic energy of the car, and it would slow down, thus requiring more energy to be put into the car. Please explain your answers thouroughly.