Heating degree day

Related measurements include the cooling degree day (CDD), which quantifies energy demand for air conditioning.

(For cooling degree days, the process works in reverse: the base temperature is subtracted from the average, and if this value is positive, that number represents the CDD.)

This more accurate algorithm enables results to be computed in temperate climates (maritime as well as continental) throughout the year (not just during a defined heating season) and on a weekly as well as monthly basis.

HDD can be added over periods of time to provide a rough estimate of seasonal heating requirements.

For example, in southern California, during winter heating is not necessary in Los Angeles and San Diego if the insulation is sufficient to take into account the colder night temperatures.

Also, buildings include thermal mass such as concrete, that is able to store energy of the sun absorbed in daytime.

HDD provides a simple metric for quantifying the amount of heating that buildings in a particular location need over a certain period (e.g. a particular month or year).

Say we are given the number of heating degree days D in one year and we wish to calculate the energy required by a building.

This gives the buildings' specific heat loss rate Pspecific, generally given in watts per kelvin (W/K).

Heat requirements are not linear with temperature,[8] and heavily insulated buildings have a lower "balance point".