A hybrid propulsion system has been designed for an optional use in the Formula S2000, based on supercapacitive energy storage coupled to an ICE engine via a Synchronous PM machine. Because of the high level specification of the weight penalty in competition, the system has been modelled and designed from different aspects of the solicitation of the used elements. After the choice of the system concept and a first evaluation of the needed performance of the electromechanical machine and converter, the electric power constraint of the supercapacitive storage elements under given energy efficiency conditions has been evaluated, as well as the thermal conditions of use.