During the past two decades, economic and environmental issues have resulted in the solar energy technology to significantly grow and develop in different fields. One of the applications of solar energy is in residential cooling and heating systems. Collectors are the principal components of a solar energy system, absorbing the energy of the sun and conveying it to the fluid with the least amount of heat dissipation and loss. The most widely used type of these is the flat plate solar collector. In the first stage of this research, in the experimental section, we tried to assess the factors affecting the flat plate solar collectors in order to increase their efficiency and improve their performance; parameters such as input and output temperatures of the fluid, ambient temperature, environmental radiation intensity, and their effect on the collector were investigated, and the flat plate collector efficiency charts were plotted. The results showed that with an increase in the input temperature fluid to the collector, the collector efficiency decreased, while an increase in the radiation intensity and ambient temperature resulted in an increase in the efficiency. In the numerical section, flat plate collectors were simulated using the EES software. To assure the validity of the results, efficiency charts were also plotted by the software. The average error between results of simulation using laboratory data were reported to be about 8%. Then, various environmental and physical parameters affecting the collectorwere assessed in order to improve its performance. Parameters such as radiation intensity, installation angle of the collector, wind speed, visible light transmittance of the glass, and absorption and admittance coefficient of the absorbing panel, posterior and lateral insulation thickness, and raiser space were investigated. The higher quality of modeling shows that EES is a powerful tool to analyze the flatbed solar collectors. Keywords: solar energy, flat plate solar collector, efficiency, simulation, EES