A probabilistic approach to the problem of life evaluation of power capacitors subjected to non-sinusoidal voltage is presented. In order to account for the stochastic nature of harmonic distortion, a probability density function representing the random occurrency of non-sinusoidal voltages is employed, while the stress-failure time relationship is provided by a multi-stress life model appropriate for life evaluation of a polymeric insulation. In such a way, the knowledge of harmonic amplitude distributions provides that of stresses affecting insulation life, i.e. electric field and temperature. Thus, life evaluation under distorted supply voltage is allowed, and comparison can be made with respect to performance in sinusoidal conditions.