degreeday

Degree day is a weather-based indicator used in energy planning and meteorology to quantify how outdoor temperatures affect heating and cooling requirements. The concept is typically divided into heating degree days (HDD) and cooling degree days (CDD). Degree days are used to estimate energy demand, compare climate impacts, and normalize energy use across different weather patterns.

Calculation is performed for each day by comparing the daily mean temperature to a base temperature. HDD = max(0, baseTemp − meanTemp) and CDD = max(0, meanTemp − baseTemp). The base temperature reflects the outdoor temperature at which building heating or cooling becomes significant energy use. A common default is 65°F (18°C) for many residential analyses, though the base can vary by climate, building type, and study purpose. For example, a day with a mean temperature of 60°F yields 5 HDD; a day with 78°F yields 13 CDD (using a 65°F base). The degree days are then summed over a period, such as a month or year.

Applications of degree days include forecasting and normalizing energy demand for utilities, budgeting and load forecasting in buildings, and analyzing climate trends related to heating or cooling needs. They offer a simple, interpretable metric that abstracts weather into a form useful for energy planning, though they are a proxy and do not capture building-specific factors like insulation, occupancy, or equipment efficiency.

Degree day data are derived from weather records and are available in daily, monthly, and annual formats, with units and base temperatures varying by source. See also: heating degree day, cooling degree day, degree-day analysis.