We consume energy in dozens of forms. Yet virtually all of the energy we use originates in the power of the atom. Nuclear fusionreactions energize stars, including the Sun, and the resulting sunlight has profound effects on our planet.

Sunlight contains a surprisingly large amount of energy. On average, even after passing through hundreds of kilometers of air on a clear day, solar radiation reaches Earth with enough energy in a single square meter to run a mid-size desktop computer—if all the sunlight could be captured and converted to electricity. Photovoltaic and solar thermal technologies harvest some of that energy now and will grow in both usage and efficiency in the future.

The Sun’s energy warms the planet’s surface, powering titanic transfers of heat and pressure in weather patterns and ocean currents. The resulting air currents drive wind turbines. Solar energyalso evaporates water that falls as rain and builds up behind dams, where its motion is used to generate electricity via hydropower.

Most Americans, however, use solar energy in its secondhand form: fossil fuels. When sunlight strikes a plant, some of the energy is trapped through photosynthesis and is stored in chemical bonds as the plant grows. Of course we can recover that energy directly months or years later by burning plant products such as wood, which breaks the bonds and releases energy as heat and light. More often, though, we use the stored energy in the much more concentrated forms that result when organic matter, after millions of years of geological and chemical activity underground, turns into coal, oil, or natural gas. Either way, we’re reclaiming the power of sunlight.

http://needtoknow.nas.edu/energy/energy-sources/the-sun/