The paper presents the use of phase change material (PCM) in window application that was studied in the framework of the ERA-NET-LAC Project “Solar Hybrid Translucent Component for Thermal Energy Storage in Buildings” (SOLTREN). Increase of energy savings in buildings is the main aim of this study. To achieve the aim, it has been decided to create a theoretical model of the impact of the PCM on the characteristics of the window and, at the next stage, to experimentally test the model. Paraffin has been chosen as the most suitable and available material as PCM. Pure paraffin has high transparency in liquid phase, and dull white colour in solid phase. The paper shows results of total transmittance and absorption spectra for different thicknesses of paraffin with a melting temperature of 35 °C, as well as absorption of the glass sample with PCM at different angles of incidence of light.