This paper describes the results of a study of Si and thermopile solar irradiance sensors over a period of almost one year. The study centered around two main topics: 1) the method of calibrating them outdoors and 2) the accuracy of such devices relative to a selected thermopile as the reference. A new method for outdoor calibration of solar irradiance sensors is proposed for adoption as an international standard. It is concluded that Si sensors are good substitutes to the more expensive thermopile pyranometers for a continuous monitoring of solar irradiance. Based on this study, the paper also proposes basic construction criteria for the class of sensors for continuous outdoor use.