Cooling degree days are a standardized measurement used to estimate cooling energy demand for a given location over a specific time period, calculated based on how far and how long outdoor temperatures rise above a reference temperature, typically sixty-five degrees Fahrenheit.
This metric provides a way to quantify and compare relative cooling demand between different locations or different years at the same location, since a location with consistently hot summers will accumulate considerably more cooling degree days than a milder climate over the same period.
Utility companies, energy modelers, and building scientists use cooling degree day data to help predict cooling energy consumption, size air conditioning equipment appropriately, and understand how a specific location’s climate affects overall cooling energy demand throughout the warmer months.
What Do Cooling Degree Days Measure?
- Cumulative departure of outdoor temperature above a reference point
- Total cooling energy demand indicators for a location or period
- Comparability between different climates or different years
- A tool for cooling system sizing and energy modeling
Why Are Cooling Degree Days Important?
Understanding cooling degree days helps explain why cooling energy costs vary so significantly between different climate regions, giving homeowners and builders a standardized way to compare and predict cooling demand based on actual, measured climate data rather than general assumptions about regional weather patterns. This standardized metric supports more informed decisions about insulation levels and cooling system sizing appropriate to a specific location.
Because cooling degree day totals can vary meaningfully from year to year even in the same location, particularly with increasingly variable weather patterns, energy modelers often use long-term average data rather than a single year’s figures to make more reliable predictions about typical cooling energy demand for design and sizing purposes.
Cooling Degree Days vs. Heating Degree Days
Cooling degree days measure how far temperatures rise above a reference point, indicating cooling demand. Heating degree days measure how far temperatures fall below a reference point, indicating heating demand, the complementary metric for cold weather energy needs.