In this paper we present a strategy to combine supercapacitors with the battery in a 500 kVA rated uninterruptible power supply. First the sizing of the supercapacitors and battery taking into account the UPS specifications is presented. The validation of the supercapacitor and the battery models is carried by using MATLAB/SIMULINK. Second, the power sharing system between the supercapacitors and the battery is presented. The supercapacitors are added as higher energy storage to overcome the full load power during short time grid failures and to boost peak power during backup time of few minutes. The UPS waveforms obtained by simulation are presented and analyzed.