This paper presents an instantaneous optimization algorithm based on the knowledge of the efficiency maps of the internal combustion engine (ICE) and the generator for the energy management system in hybrid electric vehicles. The proposed method formulates a new cost function representing the analytical expression of the overall energy efficiency of the hybrid energy source (i.e. ICE/generator set + battery pack) which is calculated based on the energy flow at the DC bus. Engine operating points are determined by assessing not only the efficiency map of the engine but also the efficiency map of the generator and the charge/discharge efficiency of the battery pack in order to maximize the efficiency of the energy delivered from the hybrid energy source to the drive system. The performance of the proposed method is analyzed and demonstrated on a hybrid electric bus developed in MATLAB/Simulink for different driving cycle conditions and the results have been compared with alternative optimization methods such as equivalent consumption minimization strategy (ECMS), model predictive control (MPC) and dynamic programming (DP) approach. The simulation results show that the proposed method provides a competitive performance with a lower computational burden compared to the alternative methods for different state of charge (SOC) ranges and drive cycle conditions. (C) 2014 Elsevier Ltd. All rights reserved.