Flat plate collector with thin absorber is studied. Heat balance equation is solved to estimate the temperature of the absorber and its variation along the local day time. The same equation is used to determine the temperature of the working fluid. A published expression  to predict with good fitting the hourly global solar irradiance is considered as a source function for the incident solar energy. Three absorbers of different materials: Copper, Aluminum and Mica are considered. The water is considered as a working fluid. Two cooling conditions at the absorber front surface are considered. Factors affecting the efficiency are revealed.