Accurate loss estimation is fundamental to the design and maintenance of photovoltaic (PV) plants. Among the various types of losses affecting PV performance, this study focuses on mismatch losses (MsL). We introduce novel stochastic models to quantify MsL in PV strings and at the combiner box. Specifically, we leverage the statistics of the minimum of N Gaussian random variables to model the stochastic behavior of MsL under realistic PV generation scenarios. To this end, we derive both exact and approximate expressions for the expected value of this minimum. Furthermore, by adapting the proposed stochastic model for MsL in strings, we develop a new formulation to estimate MsL due to PV module degradation over time. The proposed models are numerically evaluated under different parameter configurations and compared with the deterministic approach. Additionally, the stochastic MsL model for strings is validated against the Monte Carlo (MC) method employed in PVsyst, while the MsL model for degraded PV modules is validated against real field measurements, demonstrating the applicability of the proposed approach. Notably, the results obtained from the stochastic models reveal that MsL at the string and combiner box levels depends on the number of PV modules and strings, respectively.