The spectral dependence of concentrator photovoltaic devices shows a larger and more complex behaviour than conventional photovoltaic devices due to the use of multi-junction solar cells and optical elements. The spectral factor is a widely used index for evaluating the spectral influence on the performance of conventional photovoltaics in outdoors. However, the experimental analysis of this index as a tool to evaluate the spectral influence on the performance of concentrator devices has not been undertaken and still remains unknown. The aim of this paper is to analyse the spectral factor as an index for estimating the spectral influence on the power and energy output of concentrator photovoltaic systems. The final goal is to contribute to the development of new procedures for predicting the performance of this technology under real operating conditions. To achieve this goal, two concentrator modules have been monitored over the course of two years in Southern Spain. Results show that the spectral factor has a larger and different spectral sensitivity than the power output. However, this index can be used as a good first approximation for quantifying the spectral influence on the maximum power and energy yield of a concentrator photovoltaic system under real operating conditions. (C) 2015 Elsevier Ltd. All rights reserved.