Below is an interesting chart published by the U.S. Energy and Informaton Administration, showing how the U.S. used energy in the U.S. in 2007:
The sources of energy are on the left, the sectors of the economy that consume energy are on the right. The lines connecting supply sources and demand sectors show which sectors use which sources of energy. For example, petroleum represents 39.8% of the total supply of energy in the U.S. Seventy percent of that petroleum is used for transportation. Petroleum is the source of 96% of all sources of energy for the transportation sector. The transportation sector consumes 29% of all energy consumed in the U.S.
The chart reveals how natural gas is used in the U.S.: 34% in the industrial sector, 34% in the residential and commercial sector (as fuel to heat and cool homes and buildings), 30% to generate electricity. Most electricity is used by residential and commercial buildings, so in reality electricity is an intermediate demand sector. If it is eliminated as a demand sector, 61% of total demand would show as consumed by residential and commercial buildings. Natural gas would supply 14.8% of total energy used in residential and commercial buildings, either directly for heating and cooling or indirectly through its use to generate electricity.
Unlike coal and petroleum, demand for natural gas is spread among three demand sectors — industrial, residential and commercial, and electric power. Today demand for natural gas is depressed, principally because of increased supply and low demand in the industrial and electric power sectors. If natural gas’ share of the transportation sector could be increased by conversion of vehicles to burn it, domestic supplies of natural gas could supplant imported petroleum as a source of supply for the transportation sector. For an excellent article on the use of natural gas as an alternative fuel for transportation, see Seeking Alpha’s article.