A method for generating temporal solar irradiance values for a selected area. Binary format hillshade files are generated for selected azimuth and altitude points on the Sun's path for selected time points for the area. Data in the hillshade files is reclassified into reclassified files, on basis of the selected time points relative to the solar radiation data. The reclassified files are then summed to generate a set of normalized reclassified files, each representing a selected intermediate interval. The values for each corresponding one of the cells in the set of normalized reclassified files are summed to generate an irradiance-weighted shade file. The hillshade files are summed by inclusively OR-ing corresponding values for each of the cells in each of the hillshade files to generate respective composite files for each said selected intermediate interval. The composite files are then summed to generate a summed shade/time frequency file in which each data point therein represents the frequency of repetition of corresponding cells in the hillshade files over a selected upper interval of time. Each data point value in the irradiance-weighted shade file is then divided by the corresponding data point value in the frequency file to generate a file having solar access values for the upper interval, relative to the intermediate interval, for the selected area.