How to use the heating cost from degree days calculator
- Choose what you know: the heat loss per degree of the building, or its design heat loss and the design temperature difference.
- Enter the heating degree days for the period and say whether they are in Fahrenheit or Celsius degree days.
- Enter the system efficiency (for a heat pump, the seasonal COP times 100), the energy in one unit of fuel and the price per unit.
- Read the fuel cost, the fuel used and the heat the building needs, then the table for a colder or milder period.
Formula
heat loss per degree = design heat loss / design temperature difference
heat needed (BTU) = heat loss per degree x degree days (F) x 24 x (1 - free heat share)
fuel energy = heat needed / efficiency
fuel used = fuel energy / energy in one unit; cost = fuel used x price per unit
A Celsius degree day equals 1.8 Fahrenheit degree days. The factor 24 turns a daily total of degree days into hours of heat loss.
A season of 5,000 degree days
A building loses 600 BTU per hour for each degree F. Over 5,000 Fahrenheit degree days it needs 600 x 5,000 x 24 = 72,000,000 BTU, or 72 million BTU. A gas furnace at 95 percent efficiency uses 75.79 million BTU of gas, which is 757.89 therms at 100,000 BTU each. At an example price of $1.50 per therm the cost is $1,136.84.
The same building with a design heat loss of 36,000 BTU per hour at a 60 degree difference has the same 600 BTU/h per degree. A heat pump with a seasonal COP of 3 (300 percent) needs 24 million BTU of electricity, which is 7,034 kWh at 3,412 BTU each, or $1,125.44 at $0.16 per kWh. With 20 percent more degree days, the gas cost rises to $1,364.21.
What degree days measure
A heating degree day counts how far the average outdoor temperature on a day fell below a base temperature, such as 65 degrees F. A day averaging 40 degrees F is worth 25 heating degree days. Adding them up over a period gives a number that tracks how much heating the weather asked for. A building loses heat in proportion to the temperature difference, so heat loss per degree times the degree days times 24 hours gives the heat to be replaced.
The base must match the base used to find your heat loss figure, and weather services publish totals for several bases. Past bills can calibrate the number: divide a season of fuel use by the degree days for that season to see your building's own factor. For a per-hour view of one furnace, the gas heating cost calculator fits better, and for comparing fuels, the heating fuel cost calculator. To improve the heat loss figure for a wall, see the framing factor R-value calculator.
What the estimate leaves out
The method assumes the building is held at one temperature and loses heat in proportion to the outdoor difference. Wind, sun, people, appliances, ventilation changes, thermostat setbacks and equipment that cycles on and off all change the real number, and heat pumps lose efficiency in deep cold. The free heat field lets you take off a share for sun and internal gains if you know it. Fixed charges, taxes and delivery fees are not included. The result is a planning estimate and not a quote.
Assumptions and limits
- The heat loss per degree, degree days, efficiency, energy per unit and price are example values until you replace them.
- Degree days use the same base temperature as the heat loss figure.
- Efficiency is a seasonal average for the whole system. For a heat pump enter the seasonal COP times 100.
- Free heat is a flat share of the load. Leave it at 0 if you do not know it.
Frequently asked questions
Where do I find heating degree days?
National weather services and many utility sites publish them by town and month, with the base temperature they use. Pick the totals for the same dates as your bills and the base that matches your heat loss figure.
How do I get the heat loss per degree?
A load calculation or an energy audit gives it. You can also divide a past season of useful heat (fuel energy times efficiency) by that season's degree days times 24. The design heat loss option takes a figure at one design temperature instead.
Why does the table show colder and milder periods?
Heating need follows the weather, which varies from year to year. Seeing the fuel and cost at 10 and 20 percent more or fewer degree days gives a sense of the range, not a forecast.
Can I use Celsius degree days?
Yes. Choose Celsius degree days. They are multiplied by 1.8 to match a heat loss figure in BTU per hour per degree F. If your heat loss is in watts per kelvin, convert it first by multiplying by 1.8956.