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The sun is a sphere of high temperature gases about 1.4 million kilometers in diameter. The temperature of the interior of the sun is 15 million degrees Kelvin. This high temperature, combined with a pressure that is 70 billion times higher than atmospheric pressure on earth, creates ideal conditions for fusion reactions.

The fusion reaction in the sun is hydrogen atoms combining to form helium atoms, releasing energy in the process. Energy that is released is in the form of high-energy radiation, mainly gamma rays. As this radiation migrates from the center to the outside of the solar sphere, it reacts with various elements inside the sun and is transformed into lower-energy radiation, primarily in the visible light and heat portions of the energy spectrum. The sun has been producing energy in this manner for around five billion years, and will continue to do so for several billion more.

The earth orbits the sun at a distance of 150 million kilometers. Radiation expands outward from the sun at 300,000km per second, the speed of light or any other type of electromagnetic radiation. It takes about 8 minutes for solar radiation to reach the earth.

The amount of solar energy reaching a specific location on the surface of the earth at a specific time is called 'insolation', and its value depends on several factors. If the sun is directly overhead and the sky is clear, radiation on a horizontal surface is about 1000 watts per square meter. This is the highest value insolation can have on the earth¡¯s surface except by concentrating sunlight with devices such as mirrors or lenses.