Degree days overview

What is a degree day?

A degree day is a measure of relative heating and cooling energy required by buildings. It's calculated as the difference between the average daily temperature and the balance point temperature (60 degrees). When the average daily temperature is above the balance point, the result is cooling degree days; when below, the result is heating degree days.

Example 1

Average daily temperature = 80. Balance point = 60. Cooling degree days = 20 CDD. (80-60=20)

Example 2

Average daily temperature = 45. Balance point = 60. Heating degree days = 15 HDD. (60-45=15)

Example 3

Average daily temperature = 60. Balance point = 60. No degree days.

"Why not use average temperature instead of degree days?"

The problem with average temperature is highs and lows cancel each other out. A warm day (80 average temp) combined with a cold day (40 average temp) average 60. So do two mild days of 59 and 61. But in the first case there are 20 CDD and 20 HDD while in the second there is 1 CDD and 1 HDD. Using degree days, you can see the relative amount of energy required for the first set of days is much greater than for the second set of days. But if all you looked at is the average temperature, you would conclude both sets of days are the same.

What is the balance point temperature?

The balance point temperature is the average daily outside temperature at which a building maintains a comfortable indoor temperature without heating or cooling. At this outside temperature, the indoor heat gains (caused by people, lighting, equipment, etc) balance with heat loss through windows, walls, roof and ventilation.

Because the average daily outside temperature normally occurs at about 11 a.m., here’s the question: On a typical day, if the outside temperature at 11 a.m. is 60, is the building being heated, cooled or neither? If heated, then the balance point should be set HIGHER than 60; if cooled, then the balance point should be set LOWER than 60; if neither, then a balance point setting of 60 is appropriate. See the next FAQ below for a further discussion of the balance point.

The balance point temperature is set with Cooling Needed Above and Heating Needed Below meter settings in use versus weather.

Why is a 55-degree balance point better than 65?

The 65-degree balance point standard was developed 75 years ago to help the gas industry predict heating loads in residences. Studies back then showed when the average daily temperature fell below 65, residences began turning on the heat. To this day, many sources still track degree days using this standard, including the National Oceanic and Atmospheric Administration (NOAA).

Today's residences and commercial/institutional buildings are very different. Not only are walls, roofs and windows insulated much better, but also many more sources of internal heat gains exist (lights and equipment). Millions of degree day correlations by thousands of UtilityManagement users since 1982 have shown a 55-degree balance point for modern buildings is almost universally more appropriate than 65.

Use a higher balance point in non-residential buildings, use a higher balance point (56-60+) for buildings with low internal heat gains, high ventilation rates and poor insulation.