The weight of a flywheel energy storage device can vary significantly based on several factors: 1. Materials used in construction, 3. The average weight can range from a few hundred kilograms to several. Flywheel Energy Storage Systems (FESS) rely on a mechanical working principle: An electric motor is used to spin a rotor of high inertia up to 20,000-50,000 rpm. For discharging, the motor acts as a generator, braking the rotor to. Fly wheels store energy in mechanical rotational energyto be then converted into the required power form when required. It delivers full power in. However, wind and solar power's intermittent nature prevents them from be-ing independent and reliable energy sources for micro-grids.