Our study of thermal solar collectors' behaviour in a transitory regime covered a period of 9 months, from August 2007 to May 2008. The data were measured using two pyranometers (one shaded) and were recorded into a database with a baud rate of 50 seconds. Sun a position was calculated using a previously reported mathematical model embedded into an online software application. Standard technical parameters of solar thermal collectors were used to estimate their efficiency at the measuring location. Results reflect the particular conditions at Cluj-Napoca, Romania, and provide a local solar energy potential evaluation. Influences of a collector's type and time of year on thermal solar collectors' efficiency, thermal power, and accumulated heat are given. The described methodology and procedure of assessment are general and may be applied anywhere.