Filling window units with phase change material (PCM) improves the thermal performance of windows, but on the other hand it has a deteriorative effect on the optical performance due to poor heat conductivity of PCM. A novel method to tackle this drawback of PCM is to disperse nanoparticles in the PCM. In this study, a model was developed to evaluate the thermal and optical performances of window units filled with nanoparticle enhanced PCM (NePCM). The effect of different types of nanoparticles, volume fractions of nanoparticles and sizes of nanoparticles on the thermal and optical performances of windows such as temperature, heat flux, solar transmittance, absorptance and reflectance were numerically investigated and compared with the referent case (i.e. pure PCM). The results showed that the optical and thermal performances of window units filled with nanoparticle dispersed paraffin wax are improved compared to that of with pure paraffin. However, the improvement is nearly the same regardless of nanoparticle type. The effect of volume fraction and size of nanoparticle is significant during the sunset and sunrise periods. Considering both thermal and optical performances of window units, it is recommended to disperse CuO nanoparticles with the volume fraction of below 1% and nanoparticle size of below 15 nm in PCM. (C) 2018 Elsevier Ltd. All rights reserved.