This paper offers analytic calculations of the effect of the angle of inclination on the operating efficiency. Many low-potential flat solar devices such as air and water heaters, solar refrigerators, solar radiative drying installations (SRDI), and others operate with a natural-convection loop, whose processes have yet to be adequately investigated. The intensity of the heat-transfer agent for the case of free convection in a flat solar absorber is shown to depend on the influx of solar radiation, the operating temperature conditions, and the configuration of the absorber itself.