Heating Degree Days
A heating degree day (HDD) is a measurement designed to track energy use. It is the number of degrees that a day's average temperature is below 18°C (65°F). Daily degree days are accumulated to obtain annual values.
Heating degree days can describe the overall need for heating. For example, the highest temperature in a day was 1°C and the lowest temperature was -4°C. Then, the mean temperature for that day was (1-4)/2 =-1.5. The result is below 18°C, so the heating degree days are 18-(-1.5) = 19.5°C.