Outdoor spectral measurements in sub-Sahara, South Africa in particular have not been documented probably due to lack of data or lack of proper methodologies for quantifying the spectral effects on photovoltaic performance parameters. Crystalline-Si modules are widely used for system designs in most cases based on the data provided from indoor measurements or from maritime northern hemispheric conditions. As a result of this, PV systems fail to deliver their intended maximum power output. In this study, a methodology for quantifying outdoor spectral effects of c-Si modules commonly found in the African continent is presented. The results of three crystalline-Si modules indicate that these modules are affected as the spectrum shifts during seasons although these devices are perceived (without outdoor data) that their performance is not influenced by the seasonal changes in outdoor spectrum. Copyright © 2010 John Wiley & Sons, Ltd.