Solar-PV is the most mature technology for the solar energy utilization. The design of such PV-based systems are done in terms of the rated power of the panels while the rating is determined at the standard test condition of a specified temperature, irradiance level and for the standard AM1.5G solar spectrum. However, such rated output is never achieved as the actual operating conditions vary widely from the standard one. Hence, it is critical to estimate the actual power output under certain operating conditions in a given location for a proper design. Here, we provide a comprehensive method for estimating the actual power output which is applied to a particular location (Kanpur, India). Three major factors are computed which cause deviation of the power output from the rated value: spectral factor, temperature factor and irradiance factor. The combined effect is expressed through power factor which is observed to be dominated by the irradiance factor. It is observed that the actual power is significantly less from 40% to 75% of the rated value for the whole year which needs to be accounted for during the sizing-decision. The current study applies the estimation method on five commonly used PV materials and identifies amorphous silicon panel to be best suited for Kanpur for the prevailing condition in spring and summer seasons because it provides the highest output for the same rated power. The case study of Kanpur clearly demonstrates the effectiveness of the present method and its usefulness in the design process.