Também já tive esta dúvida e pesquisei em tempos.

Pode ser muito útil para quem tem aquecimento central ou ar condicionado em casa. Não estou atento a estes valores porque não sei que temperaturas devo usar para calcular correctamente os valores.

Retirado do usatoday.com

"Q: What is the formula used to calculate degree days? Are they used in any other way than forecasting heating oil requirements? Thanks.

A: To calculate the heating degree days for a particular day, find the day's average temperature by adding the day's high and low temperatures and dividing by two. If the number is above 65, there are no heating degree days that day. If the number is less than 65, subtract it from 65 to find the number of heating degree days.

For example, if the day's high temperature is 60 and the low is 40, the average temperature is 50 degrees. 65 minus 50 is 15 heating degree days.

With the forecast of a day's highs and lows you could use them to calculate the need for fuel oil or other source of energy to heat a building, if you had the necessary information about what it takes to heat the building.

Degree days are a way to obtain this information and then use it to see if investments in improvements such as more insulation or a new furnace are paying off.

To do this, you compare heating degree days with your energy bills, or the amount of energy you are using — such as therms of natural gas, gallons of heating oil, or kilowatt hours of electricity.

To do this, you need to note the dates when the gas or electric meter was read and then find the number of degree days during that period.

To find the heating degree days, go to the National Weather Service's list of local offices and click on the name of the office nearest to you. Once on the office's Web site, look on the left side of the page for a heading "Climate." and under it something like "Past weather data."

You need to find the form used to keep track of each month's weather, day-by-day. It's the NWS F-6 form, and might be labeled "monthly report."

On this form, the first column is the day of the month, and the next five are: the day's maximum, minimum, average, departure (or difference) from the day's normal, heating degree days (HDD) and cooling degree days (CDD). The month's heating degree total is at the bottom of the sheet in the HDD column.

Now, say you filled your heating fuel oil tank on Nov. 5, and on Dec. 5, you needed 100 gallons to fill it again. In other words from Nov. 5 to Dec. 5, you needed 100 gallons of oil to heat your house. You'd get the November heating degree day total and then subtract the total for the first five days. You'd then add the total for the first four days of December for the total of the degree days during the time you used the 100 gallons of oil.

Let's say that this adds up to 800 heating degree days. If you divide this into the 100 gallons of oil, you get 0.125. In other words, you used 0.125 gallons of oil per heating degree day. The longer the time you use for this calculation, the better your baseline figure is going to be.

Since each weather office Web site I've looked at has the F-6 forms going back at least a couple of years. You could get the heating degree days for the last heating season. If you have records of how much fuel you used during that season, you'd be able to calculate how much you used for each heating degree day.

Say you replaced your furnace over the winter, you could then compare this year's fuel per heating degree day figures with last year's to see if the furnace is saving you money.

The beauty of using heating degree days for this is that it makes no difference whether this winter is colder or warmer than last.

There are also cooling degree days, which are calculated the same way except you subtract 65 from the day's average temperature. These, of course, can be used to see whether your new air conditioner is saving you money.

You can use these calculations to see what any change, such as adding a room to your house, is doing to the cost of heating or cooling it."