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, reflecting climate change.

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

This graph shows trends in statewide annual cooling degree days, indicating how much warmer average daily temperatures were compared to a reference temperature of 65°F, from 1895 to 2023. Each value is the sum of cooling degree days for that year.*  

*Degree day calculations use weighted-average values based on weather station data from across California. 

Heating degree days in this graph estimate how much cooler the average temperature is than a reference temperature of 65°F. This graph shows annual heating degree days from 1895 to 2021 2023. Each value is the sum of degree days for that year.* 

*Degree day calculations use weighted-average values based on weather station data from across California. 

• Annual cooling degree days in California increased between 1895 and 2023. While 2023 weather is an exception, statewide average heating degree days have gradually decreased over the past 60 years.  Within California, changes in degree days vary locally.  

• Tracking degree days informs utility planning and construction decisions.  A key policy concern with increasing summer temperatures is that many households may not be able to cool their homes adequately to protect health and well-being. This is a particularly important issue for lower-income households, who may not be able to afford the energy costs of cooling, or even have satisfactory cooling equipment available. 

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

As California air temperatures warm, the energy needed to cool houses and buildings may also rise. 

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