The sun is regarded as the main source of energy. why ?

The sun is regarded as the main source of energy because it supplies most of our energy that we use today, directly or indirectly. Directly it provides us heat and light energy which we can use for various purpose and even can change into electrical energy by using solar cells. Similarly, indirect supply of energy is to create wind energy, hydro energy, fossil fuel energy, tidal energy. 

The sun generates energy from a process called nuclear fusion. During nuclear fusion, the high pressure and temperature in the sun's core cause nuclei to separate from their electrons. Hydrogen nuclei fuse to form one helium atom.All of the energy from the Sun that reaches the Earth arrives as solar radiation, part of a large collection of energy called the electromagnetic radiation spectrum. Solar radiation includes visible light, ultraviolet light, infrared, radio waves, X-rays, and gamma rays. Radiation is one way to transfer heat.

It radiates light and heat, or solar energy, which makes it possible for life to exist on Earth. Plants need sunlight to grow. Animals, including humans, need plants for food and the oxygen they produce. Without heat from the sun, Earth would freeze.The Sun is the Ultimate Source of Energy. Every morning the sun rises, bringing light and heat to the earth, and every evening it sets. ... Plants convert light energy from the sun into chemical energy (food) by the process of photosynthesis. Animals eat plants and use that same chemical energy for all their activities.