Hybrid electric vehicles have proven to be an effective solution for the auto industry to satisfy the increasingly stringent CO2 regulations for the short-medium term. Proper sizing of the different components is required to benefit from the hybrid architecture full potential. This paper proposes a unified method that can be applied in both cases. The method uses the energy flow, storage, and consumption during a cycle to perform the sizing. A 350 V P2 plug-in hybrid and a 48 V P2 mild hybrid are taken as a case study. The sizing is performed by adopting the WLTP cycle and subsequently analyzing the energy profile.