An investigation of estimating daily clear sky insolation 1 has been conducted. Some discrepancies have been found between the conventional method of estimation and the data. it has been noticed that the conventional method under estimates the insolation. A simpler, more elegant and more accurate "universal" relationship has been derived for estimating the daily clear sky insolation as a function of the day of the year and the latitude and altitude of a given location. (C) 2003 Elsevier Ltd. All rights reserved.