The paper presents a method of optimization of nominal voltage and minimum state of charge (SoC) of a supercapacitor (SC) energy storage system (ESS) module dedicated for stationary traction application in a DC electrification system. The criteria of optimization include minimization of power losses during a charge-discharge cycle on one hand and maximization useful energy capacity on the other. The power losses are minimised by limitation of peak power values of SC cells and DC/DC converter elements. The optimization procedure is based on the simulation model of the SC module and DC/DC non-insulated converter. The purpose of the simulation model is power losses evaluation. The calculations of the annual energy and cost savings are carried out for the parameters of ESS obtained as a result of an optimization procedure.