Heating Degree Day (HDD) is the market unit for how many degree days that a day’s average temperature is under 65°F. The colder it is, the more HDD there is. The HDD is a measurement of how much energy is going to be consumed to heat a building. For example, in the United States, the baseline temperature of 65°F less the average temperature for a given day and if the average temp is 55°F, there are 10 HDDs for that day [65°F-55°F = 10 HDD]. The HDD settle for a month is the sum of HDD of that month multiplied by 20 USD.