The paper considers the problem of choosing the optimum size for on-board energy storage system (ESS) based on supercapacitors (SCs) taking into account both the braking energy and the braking power of an electrical vehicle. The authors have derived equations for calculation of the minimum SC number in the bank and the optimum depth of its discharge. The theory is exemplified by the Skoda 24Tr trolleybus. Besides, by simulation of the ESS mathematical model, the dependence of the saved braking energy vs. SC number at the optimum discharge depth has been studied. The research shows that a reduced number of SCs may be used as compromise solution between the ESS efficiency and its cost. It was found that in most cases the optimum discharge depth is much higher than 0.5 - the value recommended by SC manufacturers and often met in literature.