We propose a method to monitor the ageing and damage of capacitors based on their irreversible entropy generation rate. We overstressed several electrolytic capacitors in the range of 33 μF–100 μF and monitored their entropy generation rate S ˙ (t). We found a strong relationship between capacitor degradation and S ˙ (t). Therefore, we proposed a threshold for S ˙ (t) as an indicator of capacitor time-to-failure. This magnitude is related to both capacitor parameters and to a damage indicator such as entropy. Our method goes beyond the typical statistical laws for lifetime prediction provided by manufacturers. We validated the model as a function of capacitance, geometry, and rated voltage. Moreover, we identified different failure modes, such as heating, electrolyte dry-up and gasification from the dependence of S ˙ (T) with temperature, T. Our method was implemented in cheap electrolytic capacitors but can be easily applied to any type of capacitor, supercapacitor, battery, or fuel cell.