Predicting both copper indium diselenide (CIS) module and generator performance outdoors is a crucial issue for designers and installers who use this technology. Three simple methods addressed to predict the behavior of two modules of this technology in a sunny climate have been empirically tried. A 12-month test and measurement campaign carried out using two calibrated CIS photovoltaic (PV) modules in the city of Jaén (Spain, latitude , longitude ) has provided the necessary empirical data. Results show that the electric behavior of these CIS PV modules may be described with adequate accuracy—for PV engineering in sunny sites—assuming the fill factor remains constant through operation while short-circuit current and open circuit voltage vary linearly with incident global irradiance and cell temperature, respectively. Further, degradation has been of no concern—otherwise, this would have invalidated the above conclusions—as it stems from the small difference between the results of the initial calibration and those of a recalibration of the modules after their outdoor exposure. Additionally, high values of the module performance ratio lead to the same conclusion.