Manufacturers usually specify photovoltaic (PV) modules at standard test conditions of and , which may not be attainable anywhere in the world. Therefore, to determine the performance of PV modules at naturally occurring irradiance and ambient temperature, this paper proposes a theoretical methodology. This method could be used to predict the performance of PV modules given the irradiance and the ambient temperature at the site of installation. A novel technique that is based on the current-voltage characteristic model is used, it accounts for the variability in the environmental parameters. The irradiance and the ambient temperature of Zaria, Nigeria were used as an illustrative example, which showed the effects of the environmental conditions on the performance of PV modules.