Ongoing oil stock depletion and growing environmental concerns lead automakers to develop more efficient powertrains. Today, the most promising way forward consists in research on hybrid systems. Defining the most efficient powertrain requires a systemic design. In this paper, three main levers are used: powertrain architecture, energy management and electric components design. Different powertrain architectures (series, parallel and combined) are compared: their optimal energetic performances are calculated for different INRETS 1 driving cycles by using dynamic programming as an optimal control strategy. The most promising hybrid powertrain is the parallel one. Its behavior is more closely analyzed so as to provide technical specifications for an optimal sizing of the electric components: electric machine and battery.