A flywheel on a bus is useful because its stored energy can be used to:

1. A spinning flywheel is a store of rotational kinetic energy. 2. That store can be drawn down when the vehicle needs power. 3. The flywheel's energy can then be used to accelerate, to go up another hill, or to keep the bus from slowing down due to friction. 4. It is a mechanical rather than an electrical or thermal store. 5. Cooling and lighting draw on entirely different systems. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 10 'Rotational Motion and Angular Momentum', sections 10.1-10.7_