This study investigated how seasonal changes in cell temperature and spectral solar radiation effect the seasonal variation in photovoltaic (PV) module conversion efficiency for CdS-CdTe and two-layer tandem-amorphous silicon (for simplicity referred to as CdS and 2L A Si, respectively). First, using the measured data, the ratio of the spectral solar radiation available for solar cell utilization to the global solar radiation (denoted hereafter as "available spectral ratio"), the mean cell temperature, and the conversion efficiency for each month were obtained, and seasonal variations were analyzed. Mean cell temperature and available spectral ratio seasonal changes were found to be 9% for CdS and 16% for 2L A Si. The CdS conversion efficiency increased during summer by 7%, whereas 2L A Si exhibited a more substantial 14% variation. During summer, although the spectral ratio available to CdS increases, the conversion efficiency does not increase by the same amount, because of an increase in cell temperature. Similarly, although 2L A Si, experienced a much greater increase in available spectral ratio and had a better overall performance, there was still only a 2% variation, because of the cell temperature increases. If the basic characteristics of solar cell output for various types are compared with the variation in environmental factors, such as irradiance, cell temperature and spectral solar radiation, it is possible to calculate the precise output of a solar cell. Accordingly, it is possible to evaluate the seasonal variation in conversion efficiency for each solar cell type and utilize this information to optimize the PV power system.