The sun is a massive fusion reactor. Only a small percentage of the radiation reach the earth's surface because of its distance from the earth. The amount of energy is equal to 10,000 times the world's energy demand. The energy that the earth collects from the sun in just one hour is equal to the total energy consumed by humans in one year.
The intensity of solar radiation outside the Earth's atmosphere depends on the distance between the Sun and the Earth.
The average value is indicated as the solar constant and is: 1367 W/m2. This level of irradiance is not reached at the earth’s surface. Earth's atmosphere reduces the radiation by reflection, absorption (by ozone layer, water vapour, oxygen and carbon dioxide).