Heating degree days (HDD) is a measure that is used to estimate the amount of energy required to heat a building or a room. It is based on the difference between the daily average temperature and a set reference temperature, usually around 65 degrees Fahrenheit (18.3 degrees Celsius).

To calculate HDD, the daily average temperature for a given period is subtracted from the reference temperature. Any negative value is ignored (i.e., set to zero), as this indicates that the average temperature was above the reference temperature and no heating was required. This process is repeated for each day in the period, and the resulting values are summed to give the total HDD for that period.

In the energy industry, HDD is used to estimate the demand for heating fuel (e.g., natural gas, heating oil) during the colder months of the year. By tracking the total HDD for a given region over time, energy companies can predict when and how much heating fuel they will need to supply to their customers. This can help them to better manage their inventory, avoid shortages, and ensure that prices remain stable.

HDD can also be used to evaluate the energy efficiency of buildings and to compare the heating requirements of different regions. A building that has high HDD values (i.e., requires a lot of heating) may be less energy-efficient than one with lower values, which could indicate the need for insulation or other improvements to reduce heat loss. Regional differences in HDD can also affect the choice of heating systems or fuel types, as some may be better suited to colder climates than others.