The rate of U. S. energy consumption reflects underlying economic trends, and is a key environmental and national security concern. Transportation accounts for approximately 28 percent of U.S. energy consumption, and most of this consumption is imported petroleum. As expected, a time series plot of energy usage exhibits a highly seasonal component; the task at hand is to separate the seasonal component from the underlying trend to observe the long-term growth of that energy consumption over time. Figure 1 shows the actual monthly transportation energy use along with the underlying trend of that data.
Figure 1 - Transportation Energy Use in Quadrillion BTUs - Actual and Underlying Trend (monthly data)
The long-term trend is essentially upward, increasing approximately 3 billion BTUs per month (or an annual growth of 36 billions BTUs). There are occasionally slight dips in the trend, which highlight the infrequent changes in the longterm pattern. But, at present, there does not appear to be any change in direction of the underlying growth.
Figure 2 - Breakout of Transportation Energy Use by Month
Seasonal patterns have exhibited some change over time, however. Figure 2 depicts monthly seasonal variation. Most months show rather consistent deviations from the long-term trend; this consistency is reflected in the relatively level, or horizontal, pattern of a month over time. For example, the spring and fall months tend to be consistent patterns over time, with exceptions for the extreme consumption months of July (much higher than average usage) and February (much lower than average usage).
NOTE: BTU - British Thermal Unit. The average heat content of motor gasoline is 129,024 BTU per gallon. One quadrillion BTU is equivalent to 7.75 billion gallons of motor gasoline.