A new model of the energy selective electron (ESE) heat engine with a variable bias voltage resulting from a variable load resistance is established. Analytical expressions for the power output and efficiency of the system are derived, based on the Fermi-Dirac distribution of electrons. The general performance characteristics of the system are revealed. The effects of the energy level of the central position of the filter, chemical potential, and load resistance on the performance of the system are discussed in detail. It is found that as long as the position of the filter is suitably designed, the maximum electric current may be obtained at zero load. The optimal values of two important parameters. the energy level of the central position of the filter and chemical potential or load resistance, are calculated for differently operating states, and consequently, two important criteria on the parametric optimum design are obtained. These results obtained here may provide some guidance for the optimum design of ESE heat engines. (C) 2012 Elsevier Ltd. All rights reserved.