I am monitoring the performance of PV modules for degradation analysis. The convenient method that is prescribed in most of the literature is temperature corrected performance ratio where you correct with the temperature coefficient provided by the manufacturer. When calculating the temperature corrected performance ratio, the seasonal effects of temperature on c-Si modules is still visible. Ideally, there shouldn't be any effects of temperature after correction and the effect of spectrum on c-Si may not be very strong to impart the seasonality. This is very confusing. Can anyone explain the possible explanation behind this.