1. From space

The Sun is the energy source for photovoltaic systems. As a first approximation, the Sun can be considered a blackbody with a temperature of approximately 6,000 K. The total power it radiates is given by the Stefan-Boltzmann law and is not monochromatic; its spectral distribution follows Planck’s law. For the Sun, the peak of its emission occurs at 570 nm (in the yellow region) and is distributed across two wavelength ranges: the visible spectrum (380–780 nm) and the infrared spectrum (780 nm–1 µm); with ultraviolet radiation (200–380 nm) accounting for less than 1% of the total radiation.

This solar energy is emitted isotropically, so only a portion of it is directed toward Earth. The amount of energy per second emitted by the Sun toward Earth is approximately 1,360 W/m ²