Degree day
A daily accumulation of how far a day's mean temperature fell below, or rose above, a stated base temperature, summed across a period to estimate seasonal heating or cooling energy. The method assumes energy input scales linearly with temperature difference, which holds for a furnace but breaks down for a heat pump.
Also called heating degree day, cooling degree day, HDD, CDD
What is a heating or cooling degree day?
A degree day accumulates how far one day's mean temperature fell below or rose above a stated base temperature, summed across a season to estimate energy use. It assumes equipment efficiency does not change across the temperature range, an assumption that fails for a heat pump.
Degree day: how it works
Each day's mean temperature is compared against a base temperature, and the difference, if in the relevant direction, is accumulated as that day's degree days. Summed across a heating or cooling season and multiplied by the building's heat loss or gain rate per degree, the total gives an estimated seasonal energy figure, valid to the extent equipment efficiency stays constant across the temperature range.
Where you meet the term
- In utility bill weather-normalisation, where degree days explain how much of a bill change came from weather rather than behaviour.
- In an early-stage energy comparison between two building upgrades, where the linear assumption cancels out between the two cases being compared.
- In a seasonal cost estimate for a heat pump, where the method's linear assumption understates energy use during the coldest, least efficient hours.
How to check it
- Confirm the base temperature matches the building. Degree days published against a 65 F base are a historical convention, not necessarily the building's own balance point. A well-insulated building's actual balance point can sit well below that figure. Technician
- Check whether the equipment's efficiency actually varies across the range. A degree-day estimate is a reasonable approximation for a furnace, whose combustion efficiency changes little with outdoor temperature, and a poor one for a heat pump, whose coefficient of performance rises and falls across the same range. Technician
Degree day: what it is not
- A degree day is not a measure of energy use on its own. It is a temperature-difference accumulation that still has to be multiplied by the building's own heat loss or gain rate.
- Comparing degree-day figures from two sources without checking they used the same base temperature and reference period compares two different things that happen to share a name.
Degree day questions
What replaces degree days for an accurate heat pump estimate?
A bin method, which calculates energy separately across temperature ranges using the equipment's actual performance curve at each range, rather than a single averaged efficiency figure applied to the whole season.
Are heating and cooling degree days calculated the same way?
The accumulation method is the same, counting the difference from a base temperature in the relevant direction, but cooling degree days also interact with humidity in a way dry-bulb degree days alone do not capture.
Do degree day figures change between climate data editions?
Yes, since they are built from the same underlying station records that get rebuilt as new observation periods accumulate, so a figure from an older reference period can differ from a current one for the same location.
Where this definition comes from
Written from standards body publication, government guidance material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- ASHRAE Handbook, FundamentalsASHRAE reference volume carrying the psychrometric relationships, the climatic design condition tables and their annual percentile basis, duct and pipe fitting loss coefficients, and the bin and degree-day methods used for seasonal energy estimation.
- Heat Pump SystemsConsumer-facing explanation of air source, ductless, and geothermal heat pump operation and efficiency metrics.