- This paper proposes a new methodology for calculating the mean time between failure (MTBF) of a photovoltaic module-integrated or module-attached inverter (PV-MII). Based on a stress-factor approach, the technique invokes the usage model for the inverter to determine the statistical distribution of thermal and electrical stresses for the electrical components. The salient feature of the proposed methodology takes into account the operating environment volatility of the module-integrated (MI) electronics to calculate the MTBF of the MII. This leads to more realistic assessment of reliability than if a single worst-case operating point were used. Measured data (temperature and insolation level) is used to experimentally verify the efficacy of the proposed methodology. The results confirm that the electrolytic capacitor is the most vulnerable component with the lowest MTBF, but more importantly provide a quantified assessment of realistic MTBF under expected operating conditions rather than extrapolating a conclusion based upon a single worst-case operating point, which may have a low probability of occurrence. 2012 IEEE.