Question 1

What are heating degree days?

Accepted Answer

Heating degree days (HDD) are a measure of how cold a location is over time, calculated by summing, for each day, the number of degrees the day's average temperature falls below a base temperature — usually 65°F in the US. A day averaging 45°F contributes 20 heating degree days. Adding them over a year gives the annual HDD, a single number that captures the heating demand of the climate and is published for cities by ASHRAE and weather services.

Question 2

How do I estimate annual heating energy from degree days?

Accepted Answer

Multiply the annual heating degree days by 24 hours and by the building's overall heat-loss coefficient UA (in BTU/hr·°F): annual load = 24 × DD × UA. This gives the heat the building loses over the year in BTU. Divide by the heating system's efficiency to get the fuel input required. For example, UA = 500 with 5,000 degree days and 90% efficiency needs about 667 therms per year.

Question 3

What is the building UA value?

Accepted Answer

UA is the building's overall heat-loss coefficient — the sum of U-value times area (U·A) for every part of the envelope (walls, roof, windows, floor) plus the equivalent conductance of air infiltration and ventilation, in BTU/hr·°F. It represents how much heat the whole building loses per degree of temperature difference. A lower UA means a tighter, better-insulated building that uses less energy for the same climate.

Question 4

What base temperature is used for degree days?

Accepted Answer

The traditional base temperature is 65°F, on the assumption that internal gains from people, lights and appliances offset the heat loss until the outdoor temperature drops below 65°F, at which point heating is needed. Modern, well-insulated or high-internal-gain buildings may have a lower balance-point temperature, so degree days to other bases (60°F, 55°F) are sometimes used for a better match. Always use degree days computed to the base that fits your building.

Question 5

Can I use degree days for cooling too?

Accepted Answer

Yes — cooling degree days (CDD) sum the degrees each day's average temperature rises above the base, and the same method estimates cooling energy: cooling load ≈ 24 × CDD × UA, divided by the cooling system's efficiency (expressed as COP or SEER). Cooling estimates are less accurate than heating because solar gain, humidity and internal loads play a larger role, so degree-day cooling estimates are rougher and often supplemented by a full load model.

Question 6

How accurate is the degree-day method?

Accepted Answer

The degree-day method is a good first-order estimate, typically within 10-20% for heating in a steady-occupancy building, and it's excellent for comparing before-and-after energy for efficiency upgrades since the same assumptions cancel out. It is less accurate where solar gain, occupancy schedules or internal loads vary a lot, or for cooling. For detailed design or code compliance, an hour-by-hour energy model is used instead, with degree days serving as a quick check.

Question 7

Is this degree days calculator accurate?

Accepted Answer

It applies the standard degree-day energy equation (24 × DD × UA ÷ efficiency), so it gives a reliable first-order annual energy estimate given accurate inputs. Use published degree days for your city at the correct base temperature, a UA that reflects the actual envelope and infiltration, and the real seasonal efficiency of the heating system. For precise design, confirm with a detailed energy model.

Degree Days Energy Calculator

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Degree-Day Energy Formula

ASHRAE / Degree-Day Method

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