Output power of Solar PV modules is significantly dependent on optical properties of the glass cover which may start degrading due to dust deposition on its surface from its very first day of installation. As dust concentration in atmosphere varies over different seasons, it causes variation of dust accumulation and also the optical properties of module glass cover. A dust chamber has been developed designed and fabricated in the laboratory to assess the change in optical properties (transmittance, reflectance and absorbance) of glass with respect to the amount of Shibpur roof-top dust accumulated on it. Experiment shows that transmittance due to dust deposition reduces from 85% to 35% depending on the deposited amount. From the study of PV power plant of IIEST, Shibpur, it is seen that dust deposition is the highest during spring. Similar results are also observed in two running PV power plants in Kolkata.