The output energy of photovoltaic (PV) modules under outdoor conditions is greatly influenced by the spectral irradiance distribution of the solar spectrum. To analyze this effect on PV modules, the spectral irradiance distribution, which is one-dimensional data, has to be represented by a zero-dimensional index. The average photon energy (APE) is an index for spectral irradiance distributions, which represents the average energy per photon in a spectrum. We have previously analyzed the uniqueness of the shape of the solar spectrum in the wavelength range of 350e1050 nm, and one corresponding value of APE showed a specific shape of spectral irradiance distribution. In this study, new indexes were calculated for a limited wavelength range of 350e750 nm and multiple bands of 450e500 nm and 800 e850 nm of the solar spectrum for easy measurement and calculation. The result shows the uniqueness of new indexes to the shape of measured solar spectrum and the standard deviations were found to be quite small. This indicates that the new indexes are reasonable for representing the spectral irradiance distribution and its effect on PV performance. (C) 2014 Elsevier B. V. All rights reserved.