Since the Extended range electric vehicle (EREV) powertrain structure is based on different power sources, a key vehicle design activity is related to development of an optimal control strategy for achieving a high fuel economy potential. The central role in developing an optimized energy management strategy is related to availability of computationally-efficient, high-fidelity EREV powertrain model. This paper proposes a method for developing an extended quasi-static backward-looking EREV powertrain model, which when compared to traditional backward model accounts for powertrain transient effects through additional fuel and battery state-of-charge consumptions. The effects of powertrain transients are characterized by means of extensive simulations of dynamic forward-looking EREV powertrain model covering a wide array of possible powertrain transient scenarios. A regression technique is then applied to model powertrain transient effects functions, which are incorporated within a standard backward model, thus combining the computational efficiency of a backward model and the accuracy of a forward model. The proposed, extended backward model is validated against the forward model for the case of a series-parallel EREV powertrain, in order to illustrate its performance in terms of accuracy and computational efficiency.