An system for storing electrical energy over at least a medium term duration. The energy storage system comprises a motor assembly operatively connectable to at least one of an electrical energy source and an electrical distribution network for providing kinetic energy, a flywheel device operatively connectable to the motor assembly for storing at least one part of the kinetic energy, a generator assembly operatively connectable to the flywheel device for receiving at least one portion of the part of the kinetic energy and generating regenerated electrical energy in response thereto, and a control unit for controlling operation of the energy storage system, which enables an energy storage operating mode for storing the part of the kinetic energy into the flywheel device, and an energy supply operating mode for providing at least one portion of the regenerated energy to at least one of the electrical distribution network and an electrical appliance.