ASHRAE Handbook Fundamentals 2021 defines Heating Design Days (HDD) and Cooling Degree Days (CDD) as the sum of the difference between daily average temperatures (calculated as the average of the daily min. and max. temperatures) and the base temperature. The more extreme the outside temperature, the higher the number of degree days. A high number of degree days generally results in higher levels of energy use for space heating or cooling.
Typically, there are two base temperatures, 50° F and 65° F, that are widely used to calculate the Heating and Cooling Degree Days. Based on the ASHRAE Handbook Fundamentals 2021 recommendation, the analysis.tool uses 65° F as the base temperature for Heating and Cooling Degree Days calculations.
HDD Calculation Example:
A day with a mean temperature of 45°F has 20 HDD when the base temperature is 65° F. Two such cold days in a row have a total of 40 HDD for the two-day period.
CDD Calculation Example:
A day with a mean temperature of 85°F has 20 CDD. If the next day has a mean temperature of 95°F, it has 30 CDD. The total CDD for the two days is 50 CDD.
Within the platform, the HDD and CDD days used for calculations are automatically extracted from the EnergyPlus Weather file (typically from .stat file type) when the user enters a project location.
Example HDD and CDD values chosen for Barksdale, LA .stat weather file: