This IEA PVPS Task 13, Subtask 2.5 reports on a benchmarking study of the various approaches for calculating the Performance Loss Rate (PLR) of commercial and research photovoltaic (PV) power plants in diverse climatic zones. PLRs are calculated with data from the PV systems’ power and weather data. The PLR is used by power plant owners, operators, and investors to determine the expected power output of a PV system over its installed life. Therefore, discrepancies in various calculation methods can greatly impact the financial around a PV installation. This benchmarking study is necessary due to the inconsistency in reported PLR results based on the many different approaches currently used to calculate PLR of PV systems. This study is focused on identifying which of the various approaches produce similar results and what causes inconsistencies between these different methods.