A rotating flywheel that is brought up at its maximum speed (3000 rpm) stores how much energy? The flywheel is a solid cylinder of mass 1000 kg and a diameter of 1.0 m. ( I = MR2/2 ) Remember that the energy stored in a rotating mass is equal to one-half of the product of the "moment-of-inertia" multiplied by the square of the rotational speed.. (A good quality battery for your car will store about 2 MJ, and weighs about 50 pounds)