Measurements of the power produced by a photovoltaic (PV) system are important for technical as well as financial reasons, especially in large installations. The energy produced by a PV system is modeled through its nominal power, the incoming irradiance and the major energy loss mechanisms. These mechanisms involve the temperature of the cells, the response of a cell to low intensity light, the spectrum and polarization of the light, the deviation from maximum power point tracking, module mismatch and ohmic losses. The experimental quantities needed by the model are the solar irradiance at the plane of the array, the cell temperature and the nominal power of the system. In this work, we present part of the results of the theoretical calculations and their comparison with actual experimental measurements. (C) 2009 Elsevier Ltd. All rights reserved.