A unit used by utility companies to roughly
predict the rate at which you will use energy to
heat or
cool your home or
business.
To calculate the degree days, get the average temperature for the day by adding the day's high to the day's low and dividing by two. If this is above 65ºF the difference is the number of cooling degree days, if it is below 65ºF, the difference is the number of cooling degree days.
Your energy bill will probably roughly follow the same pattern over a season as the heating/cooling degree days.