Heating Degree Day Calculator

What this calculator does

A balance point, the average outdoor temperature, a number of days, your home’s heat loss coefficient, a fuel type, appliance efficiency and a fuel price go in. It returns degree days per day and in total, the heat the building needs, the fuel energy that takes after efficiency losses, the fuel units, the cost, the cost per day and — the most useful output — the cost per degree day.

That last figure is the only honest way to compare one winter against another.

What a degree day is

For each day, take the 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: 20 degree days. A hundred such days: 2,000 degree days.

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

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

The balance point

The convention is 65°F, and the reason is worth knowing.

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: the outdoor temperature at which free gains exactly offset heat 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. Using 65 for such a house overstates the heating requirement, sometimes substantially. 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 then found is a real property of your building.

Getting your heat loss coefficient

This is the input that determines everything, and 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. You need a past period’s fuel use and that period’s degree days:

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 is your building’s heat loss in BTU per hour per degree of temperature difference. It 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 calculation on this page is grounded in your real house.

Comparing winters honestly

The single most useful application, and the reason cost per degree day is on the results panel.

Your gas bill was $890 last winter and $1,070 this winter. Twenty percent worse. Did something break? Did rates rise? Is the insulation failing?

You cannot tell, because those were different winters.

Divide each by its degree days. If last winter ran 4,200 degree days and this one 5,300, the figures are 21.2 and 20.2 cents per degree day. Consumption actually improved by 5 percent — the bill went up because it was a colder winter, and by less than the cold 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.

Comparing fuels

Do it 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 does not create heat; it moves heat from outside to inside, and can deliver two to four times more heat energy than the electrical energy it consumes. Expressed as an efficiency, that is 200 to 400 percent, which the model here cannot represent. A heat pump’s real performance also falls as outdoor temperature drops, which is exactly when it is needed most — modelling it properly needs performance curves, not a single efficiency number.

What this leaves out

• Solar gain, which varies enormously with orientation and glazing.
• Internal gains beyond what the balance point implicitly captures.
• Wind, which drives infiltration and can dominate on a leaky building.
• Thermal mass, which delays and smooths the response.
• Setback schedules. Lowering the thermostat overnight genuinely reduces consumption, and this steady-state model does not see it.
• Domestic hot water, which is on your fuel bill and is not heating.
• Heat pump performance curves, as above.
• Cooling degree days, the same concept applied above a balance point.

For appliance-level running costs, the energy cost calculator works from wattage and hours, and the insulation calculator covers the R-value side of reducing the coefficient in the first place.