There are nuclear reactions of fusion from "hydrogen"  into "helium"  taking place in the sun. The nuclear reactions release huge energy and radiate the energy to space continuously. This kind of energy is the solar energy.

The average intensity of solar radiation on the earth orbit is 1367kw/m2, and the earth's equatorial circumference is 40,000km, so it can be worked out that the energy the earth obtains is up to 173,000TW. That is to say that it is equivalent to 5 million tons of coal that the sun radiates solar energy to the earth per minute. The nuclear reactions of fusion in the sun can last for several billion to more than ten billion years, so it can be said that the sun is a huge, long, endless energy source, and human beings depend on the energy for living.

The energy on earth, including wind energy, hydropower, ocean thermal energy, wave energy, bioenergy and some tidal energy all come from the sun. Even the fossil fuels on earth (such as coal, petroleum, natural gas, etc.) are at bottom the solar energy that has kept in storage since time immemorial, so the solar energy in a broad sense covers a vast scope, and the narrow-sensed solar energy is confined to the direct transformation of solar radiation from sunlight to heat, electricity and chemical energy.

The solar energy is a primary energy source, and it is also renewable energy. It is rich in resources without transport, which is both free for use and non-contaminative to the environment. But the solar energy has two major drawbacks: 1. the energy flow density is low; 2. the intensity is influenced by various factors (seasons, locations, climate, etc.), so it can not maintain a constant. The two major drawbacks greatly limit the effective use of solar energy.

There is a long history for human to use solar energy. In the period of Warring States, some 2,000 years ago, Chinese people knew how to use a four-side steel mirror to focus the sunlight for making a fire, and they used solar energy to dry agricultural products. In modern times, the use of solar energy becomes increasingly widespread, which includes solar thermal utilization, solar photovoltaic utilization and solar photochemical utilization etc.