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Photovoltaic (PV) system performance assessment is an important issue in determining how much energy is effectively used by PV-load systems. International Electrotechnical Commission (IEC) 61724 establishes the reference yield as the global irradiation in the plane of the PV array. Therefore, the performance ratio obtained is only valid where the PV system is located and does not allow the comparison between the performance of PV systems installed at different locations. Adopting a reference yield that is common to any location on Earth, a new absolute performance ratio (APR) is introduced. It uses the average annual irradiation in an extraterrestrial surface equipped with a Sun-Tracking System. The interest of the new index is demonstrated through a simple example were two identical standalone photovoltaic (SAPV) systems are compared. While the IEC method shows both systems to have similar performance, the APR points out that the cost of the energy produced by one system is half that produced by the other.