What Are Heating Degree Days, Exactly?

Your heating bill went up 20 percent this winter. Did something break? Did rates rise? Is the insulation failing? You cannot tell from the bill, because those were two different winters — and degree days are the unit that makes them comparable. Once you start using them, questions about energy efficiency stop being guesswork.

What degree days measure

For each day, take a balance point temperature and subtract the day's average outdoor temperature. If the result is positive, that many degree days accumulate. If negative, zero — no heating needed.

A day averaging 45°F against a 65°F balance point produces 20 degree days. A hundred such days produce 2,000.

The virtue of the unit is that it captures both how cold and for how long in a single number. A brief severe cold snap and a long mild winter can produce identical totals and identical heating requirements, and no single temperature reading tells you that.

Typical seasonal figures across the US run from a few hundred along the Gulf Coast to over 9,000 across the northern plains. NOAA publishes both historical and current data by weather station.

Why 65°F

A house is not heated only by its heating system. Sunlight through windows, appliances, lighting, cooking, hot water and the body heat of the people inside all contribute — collectively enough to hold a typical house a few degrees above outdoor temperature for free.

So a house kept at 68 or 70°F does not need active heat until outdoors falls to roughly 65. That is the balance point: where free gains exactly offset losses.

Your balance point is probably not 65. A tight, well-insulated modern house with good windows has a lower one — sometimes 55 or below — because its losses are smaller relative to its free gains. A draughty older house may be above 65.

If a calculation using 65 consistently overestimates your real fuel use, lower the balance point until it matches. What you have found is a real property of your building, and a more useful one than any published figure.

Comparing two winters honestly

This is the payoff, and it is the reason to bother.

Take your bill for each period and divide by that period's degree days. That gives cost per degree day, which is comparable across seasons.

Worked through: $890 last winter over 4,200 degree days is 21.2 cents per degree day. $1,070 this winter over 5,300 degree days is 20.2 cents.

The bill rose 20 percent. Consumption actually improved by 5 percent. The increase was weather, and by less than the weather alone would have caused.

This is how utilities, energy auditors and building managers evaluate efficiency work, and it is the only way to tell whether an improvement did anything. Insulate the attic, then compare cost per degree day before and after. Any comparison in raw dollars is comparing weather.

The degree day calculator works out the totals and reports cost per degree day directly, so the before-and-after comparison is one number.

Getting your building's heat loss figure

To turn degree days into fuel, you need one property of the building: how fast it loses heat per degree of temperature difference.

Estimating it from square footage is close to worthless — two identical-sized houses can differ by a factor of three. Work backwards from a bill instead:

1. Convert fuel used to BTU (therms × 100,000, oil gallons × 138,500, and so on).
2. Multiply by appliance efficiency to get heat actually delivered.
3. Divide by (degree days for that period × 24).

The result captures your actual insulation, your actual air leakage, your actual windows and your actual habits — everything a table cannot know. Do it once from a cold-month bill and every subsequent estimate is grounded in your real house.

Comparing fuels

Compare on cost per delivered BTU — price per unit, divided by BTU per unit, divided by appliance efficiency.

Fuel	BTU per unit
Natural gas	100,000 per therm
Propane	91,500 per gallon
Heating oil	138,500 per gallon
Electric resistance	3,412 per kWh

Natural gas is usually cheapest per delivered BTU where it is available. Electric resistance is usually the most expensive, despite being 100 percent efficient at the appliance — the efficiency loss happened at the power station.

Heat pumps break the table. A heat pump moves heat rather than creating it, and can deliver two to four times more heat energy than the electricity it consumes. As an efficiency that is 200 to 400 percent, which a single efficiency figure cannot represent — and its real performance falls as outdoor temperature drops, which is exactly when it matters most. Modelling one properly needs performance curves.

For appliance-level running costs rather than whole-house heating, the energy cost calculator works from wattage and hours, and the kilowatt-hour guide covers what a unit of electricity actually buys.

Where to spend to move the number

Degree days tell you how hard the weather pushed. Reducing what that costs means reducing the heat loss coefficient itself.

The insulation calculator covers R-values and coverage for the highest-return areas, and the attic insulation guide explains why the attic is almost always first — heat rises, and attic work is cheap and accessible relative to walls.

Air sealing usually beats adding insulation per dollar spent, and it is the thing most often skipped because it is unglamorous and hard to photograph.

What the model leaves out

A degree-day calculation is steady-state. Solar gain, internal gains beyond what the balance point implies, wind driving infiltration, thermal mass, and thermostat setback schedules all move real consumption and none of them appear.

Domestic hot water is on your fuel bill and is not heating, so it inflates any figure derived from bills alone.

Treat degree days as a tool for comparison rather than prediction. That is what they are genuinely excellent at, and it is enough to answer the question the bill raised.

For historical degree day data by station, NOAA's climate data centre publishes the records that utilities and auditors work from.