SOCOL, a realistic and versatile FORTRAN program, has been developed to estimate net solar energy collected by a solar collector per unit collection area. This program was developed to study the properties of various solar collectors. It is made useful to a wide spectrum of users by allowing them to choose any or all of 15 possible solar collector types for calculation and comparison. Additional collectors can be included without undue labor. Either or both of two selective absorbers can be selected for energy collection calculations. SOCOL allows input for a third selective absorber. SOCOL is programmed to use solar radiation and surface meteorological data taken from The National Solar Radiation Data Base (NSRDB) for 239 stations over the USA. It can be adjusted to read other data sets. It takes 20 s on a Compaq Presario 2700 1.13 GHz computer to calculate net solar energy collection per unit area for one solar collector design using each of two selective absorbers at five fixed absorber temperatures for all the daylight hours of 1 year at one location. The program output includes sums of solar energy collection for each day, month and year along with averages and distributions. Averages and distributions for the solar radiation and surface meteorological data are also obtained so solar energy collection can be related to these data. SOCOL can be downloaded from the website: www.sci.sdsu.edu/SOCOL/. (C) 2003 Elsevier Science Ltd. All rights reserved.