The sun is the star that dominates our solar system. The amount of energy emitted by the sun as radiation is quite constant. This energy output is generated deep within the sun. As a star the sun is made up of 71% hydrogen, 27% helium, and 2% other elements. At the centre of the sun the density is 150 times that of water and the temperature is almost 16,000,000 Kelvin, which causes the nuclei of individual hydrogen atoms to undergo nuclear fusion (in other words they join together). The result of this is that two hydrogen nuclei combine to make one helium nucleus, and energy is released in the form of radiation. Vast numbers of such fusions occur every second, generating energy beyond our imagination. The energy produced moves out towards the solar surface by radiation and then by convection through the turbulent mixing of gases on the sun's surface.
The sun has produced energy for many millions of years and will do so for many millions more. It is estimated that there is enough hydrogen still in the sun's core to last another 4.5 billion years. Solar energy as we refer to it is the solar radiation (light and heat) that reaches the earth. Every day the sun radiates enormous amounts of heat and light energy. The planet and the atmosphere absorb some of this energy with the remainder being reflected back out into space.
The heat energy we receive from the sun through radiation is the energy that gives life to living things. It is also the immense source of energy that generates the planet's weather, creating wind and the planet's water cycle.