The Flow of Energy
This figure depicts the flow of energy, measured in quadrillion (1 million billion) BTUs, across the energy system of the United States for 2006, based on data from the Energy Information Administration of the U.S. Department of Energy. The chart illustrates the connections between primary energy resources (fossil, nuclear, and renewables), shown at the far left, and end-use sectors categorized into residential, commercial, industrial, and the three principal components of transportation: cars, freight, and aviation. Electricity, a carrier derived from primary resources, powers the sectors to varying degrees and is positioned closer to the middle of the chart to display its inputs and outputs. Note that hydro, wind, and solar electricity inputs are expressed using fossil-fuel plants’ heat rate to more easily account for differences between the conversion efficiency of renewables and the fuel utilization for combustion- and nuclear-driven systems. This enables hydro, wind, and solar to be counted on a similar basis as coal, natural gas, and oil. For this reason, the sum of the inputs for electricity differs slightly from the displayed total electricity output.