Prediction of output power has become important with the expansion of photovoltaic systems in recent years. Power prediction can be used for economic analysis, loss calculation and optimal designs. In this paper, a new approach for estimating the maximum output power of a photovoltaic power plant is proposed using the meteorological data under cloudy conditions. Basic concepts for understanding the behavior of modules in partial shading condition (PSC) are presented. To find the global maximum power point (GMPP), the shading index is introduced which can be used to find the maximum power point using the climate data. Then, analytical approach has been employed to find voltage and current of arrays at GMPP for both series and series-parallel configurations. The analytical approach is verified using MATLAB-SIMULINK software. Various experimental tests are made to confirm the aforementioned method validity.@en