ReRev green gyms convert the kinetic energy generated from working out into usable electrical energy. A 30-minute workout on an elliptical machine generates about 50 Watts of power, enough to charge a mobile phone 6 times or run a laptop or desktop for 1 hour or 30 minutes, respectively.
There are around 180 machines installed by ReRev green gyms around the US. Recently a set of elliptical machines installed at the University of Kansas powered the University's basketball court during a game. Talk about energy efficient workouts!