Energy efficiency is generally used as one of the most important parameters in order to introduce and compare thermal systems including flat plate solar collectors despite of the fact that the first law of thermodynamics is not solely capable of demonstrating quantitative and qualitative performance of such systems. In this paper, a theoretical and comprehensive model for energy and exergy analysis of flat plate solar collectors is presented through which the effect of the entire design parameters on performance can be examined. Upon the verification and confirmation of the model based on the experimental data, effect of parameters such as fluid flow rate and temperature, type of working fluid and thickness of the back insulation on the energy and exergy efficiency of the collector has been examined and based on the analysis and comparison of results, the optimal working condition of the system has been determined. According to the results, designing the system with inlet water temperature approximately 40 degrees more than the ambient temperature as well as a lower flow rate will enhance the overall performance. (C) 2012 Elsevier Ltd. All rights reserved.