What are heating degree days and cooling degree days?
Heating degree days are indicators of household energy consumption for space heating. It was found that for an average outdoor temperature of 65 degrees Fahrenheit, most buildings require heat to maintain a 70 degree temperature inside. Similarly, for an average outdoor temperature of 65 degrees or more, most buildings require air-conditioning to maintain a 70 degree temperature inside.
How heating and cooling degree days are computed
Take the high and low temperature for the day, and average them. If this number is greater than 65 F, then we have (Average temperature - 65) cooling degree days. If the average temperature is less than 65 degrees, then we have (65 - Average temperature) heating degree days. Running totals are kept for these units over a time period of a year so fuel distributors and power companies can assess average demands.
So it appears that the number of heating/cooling degree days accumulated on a single calendar day can be used to estimate how much energy will be required for heating/cooling a building on that day. By adding these up over time, you can figure out how much energy has been or will be used in a season. I think.