The sun is a huge ball of hot gas, which is constantly undergoing thermonuclear reactions, thus releasing huge amounts of energy. The diameter of the sun is about 1.39 million kilometers, which is 109.3 times larger than the diameter of the earth. The volume of the sun is about 1.4122×10^{7}km^{3}, which is 1.3 million times larger than the volume of the earth. The average distance between the sun and the earth is about 150 million kilometers. The energy released by the sun per second is 3.85×10^{26}, which is equivalent to the energy emitted by burning 1.32×10^{16 }tce (tons of standard coal) per second. Only about one-two billionths of the energy emitted by the sun reaches the earth’s atmosphere, which is about 173×10^{12}kW. Approximately 23% is absorbed by the atmosphere; approximately 30% is reflected by the atmosphere, dust particles and the ground back to the cosmic space; the solar radiation energy that passes through the atmosphere and reaches the earth’s surface accounts for approximately 47% (81X10^{12}kW). It is only about 17×10^{12}kw to reach the land surface, which accounts for only 10% of the solar radiation energy reaching the earth. The energy of 17×10^{12}kW is equivalent to 35,000 times the total energy consumed in the world in a year, which shows the huge potential of solar energy utilization.

Whether it is an independent power generation system or a grid-connected power generation system, all the energy comes from the sun. Generally speaking, we can only get the radiation data on the horizontal plane from the weather station, and the solar photovoltaic system’s power generation is determined by the solar cell array. The amount of radiation obtained. The amount of radiation received on the solar cell array is related to many factors: the local latitude, altitude, degree of atmospheric pollution or transparency, as well as the changes in the four seasons of the year, the changes in the time of day, and the solar radiation reaching the ground. The ratio of the straight and scattered components, the reflectivity of the ground surface, the installation and tracking of the solar cell array or the change of the inclination angle of the fixed array, and the cleanliness of the solar cell array surface. In order to calculate the amount of radiation obtained on the solar cell square array more accurately, it is necessary to have an understanding of the basic concepts of solar radiation.

The principle of direct and scattered separation of solar radiation, Burg Lambert’s law and cosine’s law are the three most basic laws.

- The principle of direct and discrete separation

The amount of radiation received on the earth’s surface (i.e. horizontal plane) and solar cell square (i.e. inclined plane) conforms to the principle of direct dispersion separation, that is, the total radiation is equal to the combination of direct radiation and scattered radiation, except that the surface of the earth receives The radiation has no ground reflection component, and the radiation received on the solar cell square array includes the ground reflection component. In addition, assuming that the scattered radiation and ground reflection are isotropic, the scattered radiation received by the solar cell array is related to the sky corresponding to the solar cell array, and the ground received by the solar cell array The reflection is related to the apparent surface of the solar cell array:

Q_{p}=S_{p}+D_{p}， Q_{T}=S_{T}+D_{T}+R_{T}

Where

Q_{p}: the total radiation received on the horizontal ground;

S_{p}: Direct radiation received by the horizontal ground;

D_{p}: scattered radiation received by the horizontal ground;

Q_{T}: the total radiation received by the inclined surface;

S_{T}: Direct radiation received by the inclined surface;

D_{T}: scattered radiation received by the inclined surface;

R_{T}: Ground radiation received by an inclined surface. - The Bouguer-Lambert law

When solar radiation passes through a certain medium, it will be weakened by the absorption and scattering of the medium. The general law of radiation attenuation by the medium can be determined by Bouguer-Lambert Low. Without considering the wavelength and atmospheric inhomogeneity, the approximate mathematical expression is:

S’_{D}=S_{o}F^{m}

Where

S_{o}: solar constant, 1350W/m’;

S_{D}: direct irradiance;

F: Atmospheric transparency;

m: air quality.

The air mass m can be calculated by the following formula:

m=1/sinα×P/p_{o}

Where

α: Sun altitude angle, which is the angle between the sun’s rays and the ground plane;

p_{o}: standard atmospheric pressure;

P: The local atmospheric pressure. - The law of cosines

The irradiance of any inclined surface is proportional to the cosine of the angle between the surface normal and the incident ray direction, which is the law of cosine.