Operational parameters of KA58 Amorphous Silicon (a-Si) thin film photovoltaic module is investigated as a function of solar irradiation and atmospheric conditions using current-voltage curves taken for a year from April to May  at Muğla University PV Outdoor Test Site. Single diode model is proposed to explain the current-voltage curves of thin film solar cells and modules. It is observed that the operational parameters of KA58 a-Si photovoltaic module exhibit significantly different response to the changes in solar radiation and weather conditions in a period of 12 months. Correlations between internal parameters, namely RS, RP, IL, IO and a (modified ideality factor) of module and solar irradiation and atmospheric data is investigated.