The power output of Photo-voltaic modules is generally rated at STC (Standard Test Condition), 1000W/m2 irradiance, 25oC temperature and 1.5G Air Mass. But in actual field condition the situation is different from STC. Output of PV module is a function of Irradiance, ambient temperature, wind speed and module temperature. It is well established that power output of PV module decreases with increase of module temperature. So it is important to know the temperature of PV module to determine the reduction in power output due to temperature. Module temperature can be determined if irradiance at that instant and NOCT value of that module is known. This work presents the variation in NOCT value with respect to rated power output of PV module and comparison of NOCT values for same rated power mono and multi crystalline Si modules from various manufacturers. According to this study NOCT value increases with the wattage of PV module. For same rated power modules, NOCT value of mono crystalline Si is more than multi crystalline Si.