In this paper, a mathematical model for the output characteristics of a photovoltaic (PV) module including shaded solar cells in series is proposed. The proposed model was developed using a general one diode solar cell model and is useful for predicting the output of a partially reverse biased module. To obtain an appropriate model for output of a PV module under partially shaded conditions, we applied three key factors to the one diode solar cell model. First, the equation for the avalanche effect is altered in order to include a parameter to relate the number of shaded solar cells to the number of non-shaded solar cells. Second, a resistance component is added in order to express the variable current originating from the avalanche effect depending on the different resistances in the PV module. Third, a compensational voltage source is included to reduce the difference in current near open circuit voltage. A special PV module that can easily change the number of solar cells in series was fabricated to validate the proposed model. By comparing the theoretical results with the measured data, we confirmed that the newly proposed equation is valid showing a root mean square error (RMSE) of less than 2.37%. (C) 2013 Elsevier Ltd. All rights reserved.