This paper presents a simple and accurate method to simulate the characteristic output of a PV system under either partial shading or mismatch conditions, that are caused by the interconnection of solar cells or modules which do not have identical properties or which experience different conditions from one another. First, a five-parameter equivalent circuit is used to represent the characteristics of a PV module or PV array under normal conditions (without partial shading or mismatch). Then an analytical method is developed to extract the five parameters using the basic manufacture template data. In this way, the IV characteristics of the PV module or array at normal operating conditions can be achieved. Additionally, the electrical characteristics of the PV module or array with bypass diodes under partial shading or mismatch conditions are analyzed in detail. To describe the IV characteristic equations of the PV module or array under complex shading conditions, a calculation algorithm with several subsection functions is proposed. Lastly, a judgment method for the states of the bypass diodes in PV modules is designed to synthesize the subsection functions. Thus, the multi-peak characteristics of a PV system under partial shading or mismatch conditions can be achieved. To evaluate the effectiveness of the proposed method, experiments have been conducted to compare the experimental and simulated IV and PV curves of a PV system under some predefined partial shading and mismatch conditions. The experimental results demonstrate that the method has high accuracy in simulating the IV and PV characteristics of a PV module or array under either partial shading or mismatch conditions. (C) 2014 Elsevier Ltd. All rights reserved.