Glossary
Degree days
A measure of how much and for how long temperature departs from a reference point, used to estimate heating and cooling demand.
Also called: heating degree days, cooling degree days, HDD, CDD
Degree days measure how far and for how long temperature departs from a reference base, typically 65°F (18°C), to estimate energy demand for heating or cooling. A heating degree day accumulates when it is colder than the base, a cooling degree day when it is warmer.
For a single day, HDD = max(0, base - average temperature) and CDD = max(0, average temperature - base), and these are summed over a billing period, month or season to produce a total. The measure is deliberately simple compared with a full numerical weather prediction output: it collapses an entire day's temperature profile into one number, trading detail for a figure that correlates well with energy use.
Utilities and energy analysts use degree days to forecast demand, normalize billed usage for weather normalization comparisons across years, and as an input to load forecasting models. The main pitfalls are using a base temperature that does not fit the building stock or climate in question, commercial and residential buildings often have different effective bases, and comparing degree-day totals across regions or providers that use different reference bases or averaging methods without checking they match.
Last reviewed September 22, 2026