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 Ultimately, virtually all of the energy on the earth comes from solar power. Wind
gets its power from the heat of the Sun. Water power
comes from the rain that is created when the Sun evaporates lakes,
streams, and oceans. Fossil fuel is stored solar power from plants that lived
millions of years ago. Even the radioactivity that drives nuclear reactors was
born in a star's supernova explosion!
This energy comes from the enormous power of the Sun, which puts out 386 billion billion megawatts
of energy, or more than a thousand billion times more energy than the US power industry's capacity in
2005. Only a very small fraction of that energy comes to the Earth; about 1100 watts per square yard
arrive at the atmosphere above the equator. This energy is reduced by cloud cover, latitude, and other
factors, so that Miami receives less than 200 watts per square yard of useable solar energy.
This energy can be captured by photovoltaic cells, special batteries that convert the power of light ("photo") into electricity ("voltaic"). In 1921, Albert Einstein was awarded the Nobel Prize in Physics for describing how the photovoltaic effect worked. He found that each photon had a specific frequency related to its energy, and that each photon could be considered an individual unit. Einstein discovered that materials such as silicon have electrons that can be knocked free if the right amount of energy is given to them. It is that property that makes solar cells work.
 As a beam of photons hits a solar cell, the photons with too much or too little
energy are reflected or pass through the cell. But the photons with just the right
amount of energy are absorbed and knock free electrons to create electricity. Because
only a fraction of the photons that hit the solar power cell have the right frequency,
only a small part of the total energy available is converted to electricity; modern solar
cells can convert as much as 1/5th of the available energy into electricity.
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