This paper illustrates the testing of a commercially available copper indium diselenide (CIS) module under real operating conditions, and the development of a semi-empirical model. The tests were carried out under different cell temperature, solar radiation and air mass conditions. A semi-empirical efficiency model was adopted to correlate It 68 measured data sets before degradation. The data was then transformed to validate the appropriateness of the model. Identical tests were repeated after one year's exposure to sunlight. The effect on the efficiency model was then investigated. In particular the change in STC efficiency and temperature coefficient will be discussed as an example of application of the efficiency model. Following detailed analysis of the results, it is suggested that the PV industry broadens disclosure of module parameters in future technical data published. (C) 2004 Elsevier Ltd. All rights reserved.