Smartening and restructuring of power industry lead to introduction of new energy resources in both supply and demand sides of energy sectors. In this regard, energy storage systems (ESSs) are appropriate alternatives for reducing the utilization of current declining non-renewable energy resources. Consequently, it is essential to consider various aspects of ESS application and face its related implementation challenges. This paper investigates the simulation of ESSs in long-term power system studies and proposes two long-term chronological load modeling methods. At first, a review of current load modeling methods in long-term studies including ESSs is provided and then two new load modeling methods are proposed. The proposed models are implemented in a typical unit commitment problem and solved for IEEE reliability test system (RTS) and IEEE 118-bus test system. Finally, a comparative study among examined load modeling methods is presented. The key feature of the proposed load models is that they are able to provide a tradeoff between computational burden and model accuracy in terms of calculating the desired requirements of the system planner. (C) 2015 Elsevier Ltd. All rights reserved.