The authors have designed a 5 MWh/100 MW superconducting flywheel energy storage plant. The plant consists of 10 flywheel modules rated at 0.5 MWh/10 MW each. Module weight is 30 t, size is /spl phi/ 3.5 m/spl times/6.5 m high. A synchronous type motor-generator is used for power input/output. Each flywheel system consists of four disk modules made from a carbon fibre composite material, rated at 125 kWh and weighing 3 t. Passive superconducting bearings, integrated into the disk modules, are used for frictionless levitation. The operating speed will be in the 2250...4500 RPM range. Maximum rim velocity is 600 m/s with a maximum tensile stress of 810 MPa. Details of the system layout are discussed, addressing thermal design and subsystems such as power conditioning system and cooling facilities. A preliminary cost analysis has been worked out. In the Western European UCPTE (Union for the Coordination of Production and Transport of Electricity) high power system, the plant could be used to provide spinning reserve for about 30 s while conventional thermal generating units ramp up to meet the increased load during peak power periods. Other applications in the electric utility field include power conditioning and stabilization in portions of a high-voltage power system.