Our Energy Budget


Earth's Energy Budget

The total energy from the sun that reaches the upper atmosphere is approximately 54.4 x 1020 kJ per year. Nearly all of this is either reflected for radiated back into space. The picture below from the National Oceanic and Atmospheric Administration shows the solar energy budget.



Solar energy warms the land, ocean, and the atmosphere. A very small amount of the solar energy reaching the Earth is converted into electricity through photovoltaic cells. These are devices that use silicon or other materials to convert sunlight directly into electricity.

Indirectly, the sun is the source of:
  1. Wind power
      Solar heating of the atmosphere causes wind. Hot air is less dense than cold air. As the hot air rises, the cold air rushes in to replace it. The wind is a principle cause of ocean waves.


  2. Hydroelectric power
      Approximately 12.5% of the total solar energy powers the water cycle. Evaporation of water from the ocean and condensation on land provides the potential energy of water in rivers for hydropower.


  3. Biofuels
      Photosynthesis is not very efficient but 0.085% of the solar energy is absorbed by plants and used to form more plant material. Approximately 450 kJ of the sun's energy is stored for every mole of carbon in carbohydrates.


  4. Fossil fuels
      A very small percentage of the plant material is buried so that it is not immediately reoxidized. A still smaller quantity of this is slowly converted to petroleum, natural gas, and coal.

There are some non-solar energy sources as well.
  • The moon influences the Earth with its gravitational force. This produces the ocean's tides. Tidal energy has been used by humans since at least the 11 century.

  • The Earth is the source of geothermal energy (heat from the interior of the planet), and the energy in the heavy, radioactive elements such as uranium-235 that we use to generate power in nuclear power plants.



US Energy Use

The US Energy Information Administration provides an annual energy review. Below is there latest data on energy in this country. It shows the source of energy and use of that energy. The units are quadrillion BTU with a total of 94.6 quadrillion BTU for 2009.



Petroleum provides 94% of energy for transportation but only about 1% for electricity. In the US as a whole, electric power comes from coal, nuclear energy, natural gas, and renewable energy. Hydroelectric power makes up most of the renewable energy electricity. In Illinois, nearly half of our electricity is from nuclear power and most of the rest is from coal-burning power plants.

In the graph below you can see how energy sources in the US have changed over time. The fossil fuels (coal, natural gas, and petroleum) remain our largest source of energy.




Professor Patricia Shapley, University of Illinois, 2012