The Impact of Spectral Irradiance Aggregation Into Kato Bands and PV-Bands on Estimates of Photovoltaic Spectral Effects

Full-resolution spectral irradiance data can comprise upwards of 2000–3000 data points at a single time step. Given that photovoltaic (PV) modeling is regularly performed with subhourly time resolutions over multiyear periods, incorporating spectral effects can become challenging both in terms of file sizes and computational burden. For this reason, spectral irradiances are sometimes aggregated into a limited number of wavelength bands (or wavebands), such as the 32-band grouping known as Kato bands that is used in the IEC 61853 series of standards on “PV module performance testing and energy rating.” To test the impact of such aggregation, measured, and modeled spectral irradiance data from two sites in the United States—Tempe, Arizona and Golden, Colorado—are used to assess the impact on PV spectral effect estimates of aggregating spectral irradiances into Kato bands and into two other sets of wavebands known as PV-bands. Calculations using the aggregated spectra are compared with those using the full-resolution spectra, for crystalline silicon and cadmium telluride modules. Each of the three sets of wavebands yields negligible errors of less than 0.1% in the spectral derate factor, which characterizes long-term spectral effects. This indicates that both Kato bands and PV-bands should be sufficient for the purposes of PV energy rating. Meanwhile, a recent version of PV-bands performs best for evaluating the instantaneous spectral mismatch factor, leading to errors of less than 0.2% across all time steps for both sites and PV module technologies, while instantaneous errors for Kato bands reach magnitudes of up to 1.4%.