Flywheel energy storage systems (FESS) are an excellent short duration grid energy storage solution; however, their cost and energy storage capacity are typical barriers to their widespread commercialization. FESS can be designed by optimizing the shape of the flywheel rotor, choice of rotor material, operating speed and rotor radius. This study optimizes the flywheel rotor shape at various operating speeds and outer radii. It is found that the energy capacity of the rotor can be improved by choosing an ideal combination of operating speed and rotor radius. Our earlier work showed that including the cost of the FESS as an optimization objective could significantly alter the FESS design . Therefore, the cost effectiveness of the FESS is also studied by comparing rotors made from different materials on an energy-per-cost basis, while the cost ratio of the materials is varied.