This paper presents a work used for a low cost method to create a real time condition monitoring and a predictive maintenance system of an electrolytic capacitor in Uninterrupted Power Supplies (UPS). It consists of evaluating the evolution laws for equivalent series resistance (ESR) and the capacitance C values of the electrolytic capacitors versus operating time. Experimental results of parameter evolutions and the law estimations are presented. The laws funded are used in UPS where waveforms are continuously varying in amplitude, frequency and versus temperature. The estimated laws are used for the on line failure prediction method which has the merits of using only the existent resources in UPS.