Over the weekend, I saw a passing reference on Twitter to the declining energy content of gasoline. Intuitively I know this to be correct for reasons I discuss below. But the poster linked to data from the Energy Information Administration (EIA) that I hadn’t previously seen.
The EIA doesn’t directly tabulate the energy content of gasoline. But they do provide two pieces of data that let us calculate it ourselves from two relevant tables in the April 2019 Monthly Energy Review.
Table 3.5 provides Petroleum Products Supplied by Type in thousands of barrels per day, while Table 3.6 provides Heat Content of Petroleum Products Supplied by Type in trillion Btus per year.
From the annual numbers, doing the appropriate conversions (which includes accounting for leap years) provides the energy content of gasoline, in BTUs per gallon, since 1949. What we find is that the EIA reported a constant energy content of gasoline from 1949 to 1992 of 125,071 Btu/gallon. I have always typically used 125,000 Btu/gal as the standard value for gasoline.
Starting in 1993, the EIA shows the energy content start to decline. The decline accelerates in 2006. What happened then? I have seen two explanations floated.
I have heard some suggest that the shale oil boom in the U.S., which created an abundance of light oil, ultimately lowered the BTU value of gasoline. This is unlikely for a couple of reasons.
First, to change the energy content of gasoline you must change the composition. As I explained in a previous article, adding butane is a recipe change that takes place seasonally. It impacts the vapor pressure of the gasoline, but it also impacts the energy content. Butane has an energy content of 103,000 BTU/gal, so the more butane, the lower the energy content of the gasoline blend. This means that winter gasoline, which contains more butane, has a lower energy content.