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In use, photovoltaic modules are exposed to a wide range of environments and mounting conditions, each with a unique combination of temperature and humidity profiles. Here, we model the temperature and humidity environments of several representative climates and use this to estimate the amount of moisture entering a PV module. Then, we use this data to estimate the hydrolytic degradation of polyethylene terephthalate (PET) used in back-sheets. A very good understanding of the degradation kinetics exists for PET. Calculations using PET kinetics along with environmental data demonstrates that the damp heat test exposes PET to hydrolytic degradation equal to hundreds or even thousands of years. However, for less understood degradation modes, we demonstrate the range of relative degradation rates likely to exist as compared to the damp heat test. This more general analysis highlights the fact that within reasonable limits a single humidity can represent a given climate. Thus when a lower representative humidity is used, one can focus testing conditions on temperature effects. This can significantly simplify testing when very little is known about the humidity dependence of degradation processes.