The unit of energy is Joule (J) or Watt-Second(Ws), but in energy markets, a very large unit of kWh is used instead because the quantities are very large.
1 kWh = 3,600,000 Ws
The quantities are however very small in solid state physics, where we evaluate how a solar cell works. The unit used is the electron volt (eV).
1 eV = 1.6 x 10-19 J
Energy is never produced, but is converted from one form to another. The largest energy consumer in a human body is the brain, and it depends on the heart to pump blood throughout the body to reach the brain. The energy for that comes from food consumption, and the energy from pumping blood is lost in the friction and resistance in blood vessels throughout the body. This is one example of energy conversion.
Energy is also used outside of the human body. In modern society, these energy usage includes heat, transport, and electricity. The demand for energy is growing due to the increase in world population and living standard. The source or supply of our energy mostly comes from fossil fuels, which are rapidly depleted, becoming difficult to extract, and produces greenhouse gases.
There are various methods of energy conversion. The basic process flows from thermal heat engines, produced by nuclear and chemical sources of energy, to electric generators. Heat energy can also be used directly in society. Electric generators can also be directly supplied with gravitational or wind energy without heat engines. Finally, there are chemical and solar sources of energy to produce electricity directly.
The most common source of energy is solar energy, directly or indirectly extracted from the Sun. The Sun causes temperature differences in the atmosphere to create winds. Waves are created by winds. Clouds and rain providing gravitational hydropower is also due to evaporation of water by sunlight. Solar cells directly extracts solar energy to produce electricity.
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