Degradation modes affecting series resistance ( R deg,modes ) are one of the major causes of performance degradation in outdoor operating photovoltaic (PV) modules. They have distinct loss incurring mechanisms under different climatic conditions. In this article, major R deg,modes have been investigated for their impact on the electrical parameters and differentiated based on their distinct electrical signatures at both cell and module levels. For this purpose, the major R deg,modes have been identified through cross-characterization of nine unaged PV modules subjected to temperature cycle, humidity freeze, and damp heat test conditions, using electroluminescence imaging and visual inspection. These R deg,modes have been broadly categorized as metallization interruption, metallization corrosion, and cracks with loss in the active cell area. The spatial characteristics of various R deg,modes have been modeled using distributed diode model of a solar cell, which has been experimentally validated. The change in fill factor has been chosen as a base metric to compare various R deg,modes . The results show that combined case of metallization interruptions can be more severe than metallization corrosion. The maximum loss in power of up to 40% has been calculated for interruption at the finger-busbar interface. Furthermore, based on the loss incurring mechanism, various R deg,modes exhibited distinct electrical signatures that were analyzed using normalized percentage change in voltage and current at the maximum power point, which has also been used to distinguish them. The distinct electrical signatures can be used for identification and nondestructive investigation of R deg,modes in the PV modules under field-operating conditions.