From the
From the photovoltaic effect to photoelectric cells

The Sun is the energy source for photovoltaic systems. To a first approximation, the Sun can be considered a black body with a temperature close to 6,000 K. The total power it radiates is given by Stefan-Boltzman's law and is not monochromatic, its spectral decomposition obeying Planck's law. In the case of the Sun, maximum emission occurs at 570 nm (in the yellow), and is divided into two wavelength ranges in the visible (380-780 nm) and infrared (780 nm-1 mm); ultraviolet radiation (200-380 nm) accounts for less than 1% of total radiation.

This solar energy is emitted isotropically, so only part of it is radiated towards the Earth. The amount of energy per second emitted by the Sun towards the Earth is approximately 1,360 kW/m²