This study examined the ASTM E28-48-13 Standard Test Method for Reporting Photovoltaic Non-Concentrator System Performance that serves as the basis of its proprietary Performance Testing procedure. It compared these methodologies to the NJR Performance Guarantees and Facility Verification Testing procedure with the intent of determining the applicability of each method to calculation of the performance capability of a given PV system of interest within a portfolio. The study used data collected via SCADA systems at six different sites over a period of seven years. The data was selected to test the methodologies, utilizing performance ratio calculations via multi-variable linear regression modeling, with various conditions that could be encountered such as non-ideal irradiance, central and distributed inverters, inverter clipping, and poor performing systems with unacceptable availability.