Both lifetime and efficiency of photovoltaic (PV) modules are negatively affected by high and/or strongly fluctuating temperatures. An approach to improve the thermal performance is to add a phase change material (PCM) to the layer composition of PV modules. In order to be able to determine design parameters of the PCM, e.g. layer thickness of the PCM, melting point or chemical composition, a mathematical 1D-model describing the transient temperature distribution across the module was developed. It considers all relevant heat transfer mechanisms – conduction, convection and radiation – as well as the melting-solidification processes of the PCM. The model is solved numerically by an iterative, implicit algorithm implemented in MATLAB. Input factors include geometrical parameters and material properties as well as time-dependent weather effects (irradiance, ambient temperature, wind speed and direction). To determine these factors, either locally measured data or values derived by empirical models from literature can be employed. The simulated temperature distribution was validated against measured temperatures with an average deviation of ±1.7 K and used as an input for a module lifetime model. The latter model showed that the integration of a PCM into the layer composition could increase the lifetime of a PV module by up to 10 y, yet at the cost of a lower energy generation which is reduced by about 12 % compared to PV modules without PCM.