All PV module’s datasheets provide I-V curve as well as the values of selected parameters at Standard Test Conditions (STC). Other parameters such as series resistance (Rs), shunt resistance (Rsh), photocurrent (Iph), diode reverse-saturation current (Io) and ideality factor (a) are not measurable. Therefore, they are not provided by manufacturers. These parameters are unique for the electrical characterization of any PV module. Based on the parameters Iph, Io, Rs, and Rsh, four equations are formed and used to obtain an exact, closed-form expression for Rs that solved numerically while increasing the value of a in small increments. To validate the proposed approach, a set of I-V curves are obtained at different values of a, and compared to the reference curve given by the manufacturer, then the parameters that corresponds to the curve that fits closest are taken. This process is done for BP SX150 and MSX60 solar modules. The proposed algorithm is compared with alternative methods available in the literature. The superiority of the proposed algorithm is verified.