The article presents the results of experimental research and a mathematical analysis of the energy efficiency of a PCM-modified transparent partition. The study was carried out during the summer season and heating season for 5 months under temperate climate conditions in Rzeszów (Poland). The solution under investigation allows for short-term storage of heat within a building window, owing to the high value of melting/freezing enthalpy of approx. 185 J/g, and the phase change material (PCM) applied. The research was conducted in parallel over two identical windows, with only one of them being modernised with a phase change thermal storage unit. The obtained results showed the possibility of improving the thermal balance of the window by 9.99%, and a more favourable adjustment of gains from solar radiation to the profile of heat demand of the adjacent room by 15.02%, compared to the reference window. The obtained results also allowed the numerical model describing the non-stationary heat exchange within the phase change material to be verified, using the solution of a Stefan problem. The obtained model was created using the equations of finite difference method in the Matlab environment. The verified model is highly compatible with empirical quantities, and constitutes a useful tool for simulating the distribution of heat storage in a PCM storage unit over time. This allows the heat gains resulting from the use of the tested storage units in the building windows to be estimated.