This paper offers an empirical approach for estimating monthly solar radiation on south-facing tilted planes with inclinations from 0 degrees to 90 degrees for building solar energy application. By measurement and the modification of previous estimated irradiation data, which have not yet been verified, the regression equations of monthly slope irradiation ratio with no ground reflection were established. The equations were validated by measurement in Taipei with correlation factor R-2 = 0.9648, and were compared with previous measurement using the Lin-estimate method. The estimated ratios of Taipei were compared with those of other cities in Taiwan. Results obtained from validation of simplified regression equations are good and practicable for building design application. Inputting the monthly the horizontal irradiation to the equations in the simplified estimative model, the monthly ratio and irradiation on the south-facing slopes can be calculated. Using the proposed calculation mechanism with local irradiation on horizontal and south-facing vertical planes to build the equations, this estimative method can also be used to conduct the local monthly irradiation on south-tilted planes for solar energy applications in other areas. (c) 2005 Elsevier Ltd. All rights reserved.