Heating Degree Days (HDD) are relative measurements of outdoor air temperature used as an index for heating energy requirements. Heating degree days are the number of degrees that the daily average temperature falls below a given heating balance point. The daily average temperature is the mean of the maximum and minimum temperatures in a 24-hour period. For example, supposing the heating balance point for a meter was 65° F, mec would calculate 25 heating degree days for a single day with a daily average temperature of 40° F.
Conversely, any day that the daily average temperature is equal to or above the heating balance point, heating degree days will be 0. If on another day the average daily temperature was 65° F or higher, heating degree days for the same meter would be 0.
Using daily temperature data and the heating balance point, mec adds up the heating degree days for each day in a billing period to arrive at the HDD values associated with individual utility bills.