Energy & costs
Heating and cooling degree days explained
Degree days measure how cold or hot a season was in a single number. Multiply by the house's heat loss per degree and you have the season's heating energy.
The idea
A house needs heat when the outdoor temperature is below a balance temperature, the point at which the heat from the sun, people and appliances is enough on its own. The traditional base is 65 °F. For each day, the heating degree days are the number of degrees by which the day’s average temperature falls below 65 °F. A day averaging 40 °F contributes 25 heating degree days; a day averaging 65 °F or warmer contributes none. Add them up over a season and you have a single number for how cold it was.
Cooling degree days work the same way above 65 °F. A day averaging 80 °F contributes 15 cooling degree days.
The US National Oceanic and Atmospheric Administration (NOAA) and the National Weather Service publish daily, monthly and annual degree days for weather stations across the country, and many utilities print them on the bill.
Typical values by climate zone
| Climate zone | Example cities | Heating degree days a year | Cooling degree days a year |
|---|---|---|---|
| 1 | Miami, Honolulu | 300 | 4,300 |
| 2 | Houston, Phoenix | 1,500 | 2,900 |
| 3 | Atlanta, Dallas | 2,800 | 1,800 |
| 4 | Washington DC, St. Louis | 4,500 | 1,200 |
| 5 | Chicago, Denver | 6,300 | 800 |
| 6 | Minneapolis | 7,800 | 500 |
| 7 | Duluth, Fargo | 9,500 | 250 |
These are representative figures for each zone, used by this site’s calculators as defaults; for an actual address, use the nearest station’s long-term normal. The same zones and the design temperatures used for sizing are in the guide to BTU per square foot by climate zone.
From design load to season energy
A house’s design heating load is its heat loss at the design temperature. Divide by the design temperature difference and you have its heat loss per degree. Multiply by 24 hours and the season’s degree days and you have the season’s heat:
The factor 0.65 is a correction from the ASHRAE degree-day method. Design loads are calculated conservatively and without internal gains, and the 65 °F base already assumes some free heat, so the straight product overstates real use. The correction brings the estimate in line with measured consumption in typical houses.
A gas-heated house in Chicago
Design heat loss 60,000 BTU/hr, zone 5 with a 0 °F design temperature and 6,300 heating degree days, a 95% furnace, gas at $1.50 per therm.
- Heat loss per °F: 60,000 ÷ 70 = 857 BTU/hr per °F
- Season heat: 857 × 24 × 6,300 × 0.65 = 84.2 million BTU
- Gas: 84.2 million ÷ 0.95 ÷ 100,000 = 887 therms
- Cost: $1,330, about $190 a month over a seven-month season
- Fuel per degree day: 0.141 therms
The degree-day fuel estimate does this for any fuel, and the heating cost by fuel calculator runs the same season through every fuel at once.
Checking a bill
Fuel per degree day is the most useful number for a house that already has bills. Divide a month’s gas or oil use by that month’s heating degree days. If last January’s bill showed 190 therms in a month with 1,300 degree days, that is 0.146 therms per degree day, within 4% of the estimate above, so the house is behaving as its design load says.
The same ratio shows whether an improvement worked. If the ratio falls by 15% after attic insulation, the insulation saved 15%, however mild or cold the following winter was. It also catches problems: a ratio that rises from one year to the next can mean a failing furnace, a leaky duct or a thermostat change.
Cooling degree days
Cooling degree days measure the cooling season the same way, but cooling energy depends more on humidity and sun than heating does on cold, so the relationship is looser. They are still useful for comparing seasons and climates: a house in Houston, with about 2,900 cooling degree days, runs its air conditioner about three and a half times as much as the same house in Chicago, with about 800.
Degree days and comparing systems
Because season energy is proportional to degree days, a heating comparison done for one climate scales to another. The heat pump versus furnace cost calculator uses the degree-day season to compare a heat pump and a furnace on the same heat, and the guide to comparing fuels per million BTU explains the cost side.
A thermostat setback removes a share of the degree days, which is why its saving is a share of the bill. The guide to thermostat setbacks sets out the rule, and the thermostat setback savings calculator applies it.
Limits of the method
Degree days assume the house’s heat loss is a straight line with temperature, the balance point is 65 °F, and the heating system’s efficiency is constant. Well-insulated houses with large solar gains have lower balance points, around 55 to 60 °F, so the 65 °F base overstates their use. Heat pumps lose efficiency as it gets colder, so a heat pump’s season cannot be computed with one COP; the calculators use a seasonal average and a backup share for that reason. For a typical house the method is close enough for budgeting and for comparing fuels, and a season of bills, divided by its degree days, shows how close it is for yours.