“Degree days” is a measure of energy needed to cool or heat building spaces to a comfortable temperature. It is useful for planning energy costs for different locations, and is far superior than using average temperatures in which highs and lows tend to cancel eachother. The link below to the Numbers column of the Wall Street Journal is the best explanation I have seen on the subject. The second link is to a U. S. government website which is also very helpful.
How Do Energy Companies Measure the Tem…re? Not in Fahrenheit or Celsius – WSJ
https://www.eia.gov/energyexplained/index.php?page=about_degree_days
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