A recent headline highlighted that the U.S. produced less than 40% of its electricity from coal, which represented the lowest share for coal fired electricity since 1978. Heck, I even linked to the article because I felt that it was an important milestone.
However, it got me to thinking. When quantities are presented as a percentage of the total it does not always tell the truest story. When one sees a headline like the statement about coal fired electricity the natural thought is that the total amount of electricity powered by coal went down. In reality, the amount may have stayed flat or even increased because it is a number that is relative to several others.
Here is the total electricity generation in the U.S. for all power sources going back to January 1989:
The spikes correspond to the summer demand for electricity in the U.S. that is correlated to demand for air conditioning as the temperatures rise. The red line is merely a linear trend line that shows the general increase in total electricity generated in the past 23 years. The data set was obtained from the U.S. Energy Information Administration and is available here.
To get rid of the extreme peaks and to look at the general trends in electrical generation sources I compiled the monthly data into a twelve month rolling total. It produces a chart that looks like this:
What this chart shows graphically is the relative small size of the traditional renewables—solar and wind—compared with the legacy generation sources—coal, natural gas, and nuclear. However, the chart also shows that coal’s generation share and total generation is falling in the most recent period.
April 2008 represents the high water mark for coal fired electrical generation in this data set. For the 12 month period up to April 2008 coal fired electricity was responsible for 2,028,321 million kilowatt hours. By November of 2011 coal fired electricity had fallen to 1,768,817 million kilowatt hours for the 12 month period. This represents a 12.8% drop.
Meanwhile, the high water mark for total electricity generation was in July 2008 at 4,184,758 million kilowatt hours across all generation types. By November of 2011 the total had fallen to 4,126,167 million kilowatt hours or a drop of 1.4%, which is huge when dealing with such a large base number.
One interesting phenomenon is the decline in total electricity generation from April 2008, the high water mark for coal fired electricity generation, through December of 2009. For that period of time, the total dropped 5.4% or 226,701 million kilowatt hours. From the period ending July 2008, the high water mark for total electricity generation, dropped 5.8% or 241,728 million kilowatt hours.
This drop corresponds tightly with the deepening of the most recent recession. Correlating the unemployment rate for by month against the total electricity generation for the trailing 12 month period results in a coefficient of (67.4) which suggests a moderately strong inverse relationship between the two datasets. That is to say as unemployment goes up, electricity generation in total goes down. It only makes sense.
In summation, the good news about goal generation falling as a percentage of the total electrical generation market is good. It is tempered somewhat by some other factors that I will look at going forward.