Years ago I read an article about storing energy in High Speed Flywheels to power city buses. The flywheels would be reved up in the morning utilizing electric motors and the buses would then run all day using a clutch and gearbox system. This system is the mechanical version of the Electric Vehicle. Please don’t ask me where I found this article, I forgot…So back to the subject. A lot of power would be required to accelerate the flywheel inertia to maximum speed and less power would be used to run the buses all day. This is do to a thing called System Efficiency….So here is my Idea, what if we accelerate a hollow vertical flywheel to maximum speed (a fraction of the power), then give it its mass by filling it with water through a hole in the center of the shaft. Would the buses run on less energy from the main power source? Just looking for some comments on this one, thank you.