The energy formed inside the sun is radiated into the space with a constant intensity according to human scale, and this radiation reaches our planet as well. The solar constant is the numerical value indicating the amount of energy reaching the upper boundary of the atmosphere for a surface unit perpendicular to the propagation direction of the radiation during a time unit and at an average Earth-Sun distance. Its current value is 1,353 W/m2. Since the Earth’s orbit around the Sun is elliptical – one focal point is the Sun – the distance between the Sun and Earth constantly changes, thus the solar constant changes throughout the year between the values 1307 W/m² and 1398 W/m². The beams considered as parallel until the boundary of the atmosphere change into diffuse radiation in the atmosphere. For equipments using solar energy, calculations are made with the sum of direct radiation and diffuse radiation = total radiation, which is called global radiation. These data are related to clean atmosphere.