In this study, the exact amount of the rate of irradiance within the ambit of a tube in the evacuated tube solar collector (ETSC) has been measured in various hours of the day by means of a solar meter. Some novel correlations for determining the distribution of insolation intensity around the tube have been proposed by the analysis of experimental data. A correlation is offered by using the results of previous studies and mathematical analysis. The offered correlation is utilized to calculate the direction and the amount of the maximized insolation intensity in cross section area of the tube concerning geographical circumstances, hour angle, and the slope of the collector by denoting the solar radiation distribution function around the section of the tube. Moreover, by integrating around the tube, a correlation for calculation of the whole amount of the absorbed radiation through the absorber has been presented. In the end, the error analysis is conducted and the maximum amount of error for the proposed model is estimated to be 8% before 5 p.m., and at solar noon the amount of error is reported less than 1%. Furthermore, the energy efficiency of the constructed water heater is obtained as 65%. (C) 2018 Elsevier Ltd. All rights reserved.