Incorporating paraffin wax into double glazed windows is a novel method to reduce energy consumption and adjust indoor thermal environment of buildings. However, the low thermal conductivity of paraffin wax limits the thermal performance of glazed windows. Adding nanoparticles into phase change material (PCM) is one of the effective methods to improve thermal performance. In this study, the effect of nanoparticle volume fraction and particle diameter on the temperature difference between the glass interior surface and indoor environment, and energy consumption of glazed windows containing Nano-PCM were numerically investigated for representative days of different seasons. The developed numerical model was validated with the experimental results obtained in this work. The results show that the higher nanoparticle volume fraction, the higher energy consumption, and the larger particle diameter, the smaller energy consumption. The minimal energy consumption is obtained with nanoparticle concentration of 1% and nanoparticle diameter of 100 nm for all the seasons. The magnitude of impact of the studied parameters is more significant in winter. Energy consumption can be reduced up to 4.0% by an appropriate selection of nanoparticle volume fraction and nanoparticle size. The temperature difference between interior surface of glazed windows and indoor environment are mainly concentrated in the range of 1-4 degrees C for summer, 5-10 degrees C for autumn and 14-16 degrees C for winter.