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If you want to be faithful to the original derivation of degree-days, the equation would be (since I don't have an equation editor handy, I'll just describe it in words :-) ) to calculate the daily mean as the average of the max and min temperature of the day, and then subtract that from the base temperature (65F in the US that then got translated to 18.3 C elsewhere), and then sum the positive values over all the days.

Some tidbits on how this "classical" formulation evolved: 1. It was used to estimate the amount of coal needed in Chicago (?) in the 1930s.

1. Max and min temperatures were used because hourly temperatures were unavailable.

2. 65 F can be interpreted as the balance point temperature (BPT) of a building, below which heating would be required. As buildings have gotten tighter, there was talk 20+ years ago that lower BPTs should be used, but that became irrelevant with the proliferation of simulations, leaving 65 F degree-days as just a general climate index.

3. Using hourly values, i.e., degree hours/24, will give slightly higher values (3-8%), but these paradoxically often correlate not as well to building energy use, because they ignore building thermal inertia.

If you want to be faithful to the original derivation of degree-days, the equation would be (since I don't have an equation editor handy, I'll just describe it in words :-) ) to calculate the daily mean as the average of the max and min temperature of the day, and then subtract that from the base temperature (65F in the US that then got translated to 18.3 C elsewhere), and then sum the positive values over all the days.

Some tidbits on how this "classical" formulation evolved: 1. It was used to estimate the amount of coal needed in Chicago (?) in the 1930s.

1. Max and min temperatures were used because hourly temperatures were unavailable.

2. 65 F can be interpreted as the balance point temperature (BPT) of a building, below which heating would be required. As buildings have gotten tighter, there was talk 20+ years ago that lower BPTs should be used, but that became irrelevant with the proliferation of simulations, leaving 65 F degree-days as just a general climate index.

3. Using hourly values, i.e., degree hours/24, will give slightly higher values (3-8%), but these paradoxically often correlate not as well to building energy use, because they ignore building thermal inertia.

If you want to be faithful to the original derivation of degree-days, the equation would be to calculate the daily mean as the average of the max and min temperature of the day, and then subtract that from the base temperature (65F in the US that then got translated to 18.3 C elsewhere), and then sum the positive values over all the days.

Some tidbits on how this "classical" formulation evolved: 1. It was used to estimate the amount of coal needed in Chicago (?) in the 1930s.

1. Max and min temperatures were used because hourly temperatures were unavailable.

2. 65 F can be interpreted as the balance point temperature (BPT) of a building, below which heating would be required. As buildings have gotten tighter, there was talk 20+ years ago that lower BPTs should be used, but that became irrelevant with the proliferation of simulations, leaving 65 F degree-days as just a general climate index.

3. Using hourly values, i.e., degree hours/24, will give slightly higher values (3-8%), but these paradoxically often correlate not as well to building energy use, because they ignore building thermal inertia.

If you want to be faithful to the original derivation of degree-days, the equation would be to calculate the daily mean as the average of the max and min temperature of the day, and then subtract that from the base temperature (65F in the US that then got translated to 18.3 C elsewhere), and then sum the positive values over all the days.

Some tidbits on how this "classical" formulation evolved: 1. evolved:

1. It was used to estimate the amount of coal needed in Chicago (?) in the 1930s.

   1. Max and min temperatures were used because hourly temperatures were unavailable.

   2. 65 F can be interpreted as the balance point temperature (BPT) of a building, below which heating would be required. As buildings have gotten tighter, there was talk 20+ years ago that lower BPTs should be used, but that became irrelevant with the proliferation of simulations, leaving 65 F degree-days as just a general climate index.

   3. Using hourly values, i.e., degree hours/24, will give slightly higher values (3-8%), but these paradoxically often correlate not as well to building energy use, because they ignore building thermal inertia.

   Update:

   In answer to the previous question, I've never bothered to check if the equation has been defined in some standard, although it has been described in a lot of technical literature (just do a Google search). Frankly, I don't understand the compulsion for seeing methods standardized.

   While I'm on this subject of building energy-related climate indicators, I'd like to repeat that degree days is just the earliest indicator developed and only addresses sensible heat loss/gain, i.e., temperature. There have been similar indicators proposed for humidity (Latent Enthalpy Days), solar (Heating/Cooling Insolation Days), even infiltration (Infiltration DD's), but they've not been widely used or adopted. For a description of these indicators, please look at this old paper that I wrote 28 years ago (if nothing else, it might impress you with its age!)

If you want to be faithful to the original derivation of degree-days, the equation would be to calculate the daily mean as the average of the max and min temperature of the day, and then subtract that from the base temperature (65F in the US that then got translated to 18.3 C elsewhere), and then sum the positive values over all the days.

Some tidbits on how this "classical" formulation evolved:

1. It was used to estimate the amount of coal needed in Chicago (?) in the 1930s.

2. Max and min temperatures were used because hourly temperatures were unavailable.

3. 65 F can be interpreted as the balance point temperature (BPT) of a building, below which heating would be required. As buildings have gotten tighter, there was talk 20+ years ago that lower BPTs should be used, but that became irrelevant with the proliferation of simulations, leaving 65 F degree-days as just a general climate index.

4. Using hourly values, i.e., degree hours/24, will give slightly higher values (3-8%), but these paradoxically often correlate not as well to building energy use, because they ignore building thermal inertia.

Update:

In answer to the previous question, I've never bothered to check if the equation has been defined in some standard, although it has been described in a lot of technical literature (just do a Google search). Frankly, I don't understand the compulsion for seeing methods standardized.

While I'm on this subject of building energy-related climate indicators, I'd like to repeat that degree days is just the earliest indicator developed and only addresses sensible heat loss/gain, i.e., temperature. There have been similar indicators proposed for humidity (Latent Enthalpy Days), solar (Heating/Cooling Insolation Days), even infiltration (Infiltration DD's), but they've not been widely used or adopted. For a description of these indicators, please look at this old paper that I wrote 28 years ago (if nothing else, it might impress you with its age!)