The solar resource 

Part of the energy that the Sun radiates around it reaches the surface of the Earth. This energy comes to us essentially in the form of electromagnetic waves like those used in radio or television broadcasts, but shorter wavelengths. This radiation consists of an overlap of waves whose lengths are between 0.25 microns (1 micron = 0.001 mm = 1 um) and 4 microns. Those used in the radio range from 1 meter to a few kilometers. That is, the shorter the wavelength, the greater the energy associated with the wave; hence the danger of ultraviolet rays and other shorter waves such as X-rays.

The density of solar radiation just outside the Earth's atmosphere is 1353 W per square meter of a surface perpendicular to the sun's rays. This value is known as the solar constant; but it varies from +/- 4%, depending on the time of the year (seasons).

The atmosphere dissipates part of the solar energy that comes from the Sun:

By molecular diffusion (especially for ultraviolet radiation).

By a diffuse reflection on aerosols (dust, small drops, etc.).

By gas absorption.

The lower the sun on the horizon, the greater the layer of air the rays must cross and the less energy that reaches the ground.

When the angle formed by the horizontal plane of the place with the sun's rays becomes less than 15º, it is useless to pretend to capture these rays. The thickness of air that must pass absorbs almost all the energy.

The various actions of the atmosphere eliminate most of the solar energy, especially in the ultraviolet and infrared domain.