Cooling and heating degree days are temperature-based metrics that help estimate cooling and heating needs. In California, cooling degree days have gradually increased, and heating degree days have gradually decreased as a result of climate change.

The need to cool indoor living spaces depends on the outdoor temperature. The energy needed to cool buildings during warm weather is measured by “cooling degree days,” while the energy needed to heat buildings during cold weather is measured by “heating degree days.” For more information, download the Cooling and Heating Degree Days chapter. 

Cooling degree days measure how much the average daily temperature is higher than 65°F, indicating the energy demand for cooling. This graph shows annual cooling degree days from 1895 to 2021. Each value is the sum of degree days for that year.

Heating degree days measure how much cooler it is than 65°F, indicating the energy demand for heating. This graph shows annual heating degree days from 1895 to 2021. Each value is the sum of degree days for that year.

• Annual cooling degree days in California increased between 1895 and 2021, with the largest increases occurring in coastal areas of the state.  Annual heating degree days in California decreased, with the past few years showing unusually high and unusually low values. The Central Coast and Southern California (both coastal and inland) had the largest declines in heating degree days.

• Tracking degree days informs utility planning and construction decisions, as well as for estimating changes in biological systems such as agriculture
  Meeting the energy demand is especially important for cooling lower-income households that are more vulnerable to the health effects of heat, less likely to own air conditioners or well-functioning and energy-efficient units, and often cannot afford cooling costs.

• Since heating and cooling degree days reflect trends in temperature so the factors that influence temperature also affect this indicator. 

As California air temperatures warm, the energy needed to cool houses and buildings is rising. 

Credit: iStock/photovs

• Cal-Adapt, Cooling and heating degree days
• National Weather Service, Degree Days
• National Oceanic Atmospheric Administration, Climate at a Glance
• US Energy Information Administration, Units and calculators explained: Degree days