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The sun is a very powerful source of energy and is the only source of energy that is, for all practical purposes, infinite in supply. In the broadest sense, solar energy supports all life on Earth and is the basis for almost every form of energy we use. The sun makes plants grow, which are burned as fuel or rot in swamps and are compressed underground for millions of years to become coal and oil. Heat from the sun causes temperature differences between areas, causing the wind to blow. Water evaporates because of the sun, falls on high elevations, and rushes down to the sea, spinning turbines as it passes. But solar energy usually refers to ways this energy can be used as heat and electricity. The sun is continuously emitting enormous amounts of energy. A fraction of that energy reaches the earth. The fraction of the energy from the sun that reaches the earth in just one day is still more than enough to cover the energy use of the world in a whole year. However, not all the energy of the sun that reaches the earth can be used effectively. Part of the sunlight is absorbed in the earth's atmosphere or reflected back into space. Residential and commercial buildings account for one-third of the North American energy use, in