The work herein considers the graphic-analytical method of the flat solar collector's energetic and opticalcharacteristics. There has been computed the total solar radiation on the solar collectors' inclined surfaces.In the research methodology, as an example of the total solar radiation computation, there has been taken Astana city. According to the observation data it is seen, that the solar radiation duration falls in summer. So, by means of graphicanalytical method there has been computed the distributions of direct solar radiation flow stream density, optical performance coefficient from direct solar radiation flow stream density and total flow to the receiver.Optical performance coefficient has constituted 66%.It proves the correctness of the solar collector parameters selection. INTRODUCTIONScope of technogenic impact on the environment and sustainable demand for the power underlines the urgent transfer to the renewable energy sources. Solar thermal collectors market is 8 times bigger, than it has been twenty years ago, as the solar collector is more efficient, than a photovoltaic system for heat production [1]. Apart from that, the cheaper storage, comparable with accumulators, has made the solar thermal collectors more profitable for the heat production, than photovoltaic systems. The solar thermal collectors are usually broken down into three categories based on the heat transfer fluid (HTF) temperature. Low temperature solar collector (less than 100°C) consists of a flat plate and evacuated tube collectors (ETC), which are able to maintain the hot water and air for residential properties, high temperature collector (over 300°C) [2,3]. Such linear Fresnel collectors and parabolic ducts, are, by and large, designed for the steam generators vapor provision [4]. Finally, the less studied category represents an average temperature collector (100-300°C) with a wide spectrum application, for instance, desaltation [5], mining operations, vapor generation [6] and zero liquid [7]. Various approaches to an optical analysis have been carried out by several researches. Jose has analyzed the flow distribution in the solar furnace focal spot considering the sun limb darkening [8]. Evans has used an integral dependence to determine the intensity both of flat absorbers and, cylindric and parabolic concentrators under perfect conditions, supposing the sun disc [9]. Dali has made use of the back-beam tracking procedure to define the flow distribution according to the solar concentrator [10]. Nicolya has conducted a flow distribution Ye. N. Amirgaliyev, M. Kunelbayev, T. Merembayev, S. Daulbayev & A. Irzhanova Impact Factor (JCC): 7.6197 SCOPUS Indexed Journal NAAS Rating: 3.11 Ye. N. Amirgaliyev, M. Kunelbayev, T. Merembayev, S. Daulbayev & A. Irzhanova Impact Factor (JCC): 7.6197 SCOPUS Indexed Journal NAAS Rating: 3.11