A method is presented herein that allows the determination of the average temperature of solar cells in a concentrator photovoltaic (CPV) module. The module is measured systematically in a sun simulator while the average module temperature and the irradiation are varied. Two different approaches are discussed to heat a CPV module in a sun simulator. From the measurements, a function is derived that allows the derivation of the average solar cell temperature when the I-V curve of the CPV module is measured. Consequently, the module itself can be used as a temperature sensor. Outdoor I-V measurements of different CPV modules are then presented. Their temperatures are calculated by applying the newly developed method. A multi-linear regression is conducted on the data measured outdoors. In particular, the modules' maximum powers are correlated to direct normal irradiation, the solar spectrum and the average solar cell temperature. The impact of temperature on the module's maximum power is shown to be significantly smaller than the impact of the solar spectrum. Finally, the maximum power values for the modules are re-calculated for two different rating conditions. (C) 2011 Elsevier B.V. All rights reserved.