What degree days measure
Heating degree days (HDD) measure how cold a period was, and therefore how much heating a building needed. For each day, HDD = base temperature − average outdoor temperature, counted only when positive. The US standard base is 65 °F: below that average temperature, buildings generally need heat.
Cooling degree days (CDD) work the other way, counting how far the average temperature rises above the base. They measure air-conditioning needs.
The formula
HDD = Σ max(base − daily average temperature, 0)
For a quick estimate over a period, multiply the number of days by the difference between the base and the period’s average temperature. Summing daily values is more accurate when temperatures swing around the base.
Example
A 30-day month averaging 40 °F, with 90 therms of natural gas used for heating:
- HDD = 30 × (65 − 40) = 750;
- use per degree day: 90 ÷ 750 = 0.12 therm per HDD;
- for a normal month of 800 HDD, expect about 96 therms;
- at $1.50 per therm: $135 this month, $144 in a normal month;
- this month was 6.3% milder than normal.
Why it is useful
- Compare years fairly: a lower bill in a mild winter does not mean your house got more efficient. Use per HDD does.
- Measure improvements: compare therms or kWh per HDD before and after new insulation, windows or a heat pump.
- Budget: multiply your use per HDD by the normal HDD of the coming months.
- Detect problems: a sudden rise in use per HDD can signal a furnace issue or a thermostat change.
Where to find degree days
NOAA and the U.S. Energy Information Administration publish degree days by region, and many utilities print them on the bill. Weather websites provide daily values for your nearest station. Use the same base and the same station for every comparison.
Limits
- The 65 °F base is a convention. Well-insulated homes with internal gains from people and appliances may not need heat until 55 to 60 °F.
- Water heating and cooking also use gas. Subtract a summer month’s use to isolate heating.
- Wind, sun and humidity also affect heating needs but are not captured by degree days.