Degree Days Explained
Degree days are essentially a simplification of historical weather data - outside air temperature data to be specific. Degree-day data is easy to get hold of, and very easy to work with. This makes degree days popular amongst energy consultants and energy managers, certainly when compared with other forms of past weather data such as half-hourly temperature readings.
Degree days can come in any timescale, but they typically come as weekly or monthly figures. You can sum them together to make figures covering a longer period (e.g. sum 12 consecutive monthly degree-day figures to make an annual degree-day total). This is useful if you are working with, say, quarterly or annual energy-consumption figures.
There are two main types of degree days: heating degree days (HDD) and cooling degree days (CDD). Both types can be Celsius based or Fahrenheit based.
Heating Degree Days (HDD)
Heating degree days (HDD) are used for calculations that relate to the heating of buildings. For example, HDD can be used to normalize the energy consumption of buildings with central heating.
Heating degree-day figures come with a "base temperature," and provide a measure of how much (in degrees), and for how long (in days), the outside temperature was below that base temperature. In the UK, the most readily available heating degree days come with a base temperature of 15.5°C; in the US, it's 65°F.
An example calculation: if the outside temperature was 2 degrees below the base temperature for 2 days, there would be a total of 4 heating degree days over that period (2 degrees x 2 days = 4 degree days). In reality, the process of calculating degree days is complicated by the fact that outside temperatures vary throughout the day. Fortunately, however, you can use degree days in your own calculations without worrying about how they were calculated originally!
Cooling Degree Days (CDD)
Cooling degree days (CDD) are used for calculations relating to the cooling of buildings. For example, CDD can be used to normalize the energy consumption of buildings with air conditioning.
Cooling degree-day figures also come with a base temperature, and provide a measure of how much, and for how long, the outside temperature was above that base temperature.
Heating degree day (HDD) and cooling degree day (CDD) are quantitative indices designed to reflect the demand for energy needed to heat or cool a home or business. These indices are derived from daily temperature observations, and the heating (or cooling) requirements for a given structure at a specific location are considered to be directly proportional to the number of heating degree days at that location.
More specifically, the number of heating degrees in a day is defined as the difference between a reference value of 65°F (18°C) and the average outside temperature for that day. The value of 65°F is taken as a reference point because experience shows that if the outside temperature is this value then no heating or cooling is normally required. Occupants and equipment within a building usually add enough heat to bring the temperature up to a more comfortable level.
Suppose, for example, that the average temperature for a given day is 55°F. Since this value is ten degrees lower than the reference point of 65°F then one would say this is a ten degree-day. Obviously, the outside temperature is not always constant, so one needs a method to determine the average temperature. A simple way to do this is to compute the arithmetic mean of the high and low temperatures for the day. While not always correct, this is sufficiently accurate for most purposes and is done for practicality because these temperatures are always recorded by the weather bureau. Thus, in the previous example, if the high temperature were, say, 65°F and the low 45°F, then the average would still be 55°F for a ten degree-day.
65°F is known as the "base temperature" of the degree days in the USA. Degree days are commonly found with a base temperature of 65°F (18°C), and 15.5°C (59.9°F) is common in many countries such as the UK. However, heating and cooling degree days can actually be calculated using any base temperature - the most appropriate base temperature to use depends on the application (e.g. the temperature that the building is heated to and the amount of heat supplied by people and equipment).
Heating and cooling degree days can be added over periods of time to provide a rough estimate of seasonal heating and cooling requirements. In the course of a year, for example, the number of heating degree-days for New York City is around 5,000 whereas that for Barrow, Alaska is over 20,000. Thus, one can say that, for a given home of similar structure and insulation, four times the energy would be required to heat that home in Barrow than in New York.
The degree-day system has several problems. The base temperature with which degree days are most readily available actually varies from country to country. For example, the "default" base temperature in the UK is 15.5°C (59.9°F), whilst, in the US, it's 65°F (18°C). This alone is a pretty strong indication that the one-base-temperature-fits-all approach to degree-day-based calculations is problematic.
Heat requirements are not linear with temperature, and heavily insulated buildings have a lower base temperature or "balance point." The amount of heating and cooling required depends on several factors besides outdoor temperature: How well insulated a particular building is, the amount of solar radiation reaching the interior of a house, the number of electrical appliances running (e.g. computers raise their surrounding temperature) the amount of wind outside, number of people residing within the building, and individuals' opinions about what constitutes a comfortable indoor temperature. Another important factor is the amount of relative humidity indoors; this is important in determining how comfortable an individual will be.
These sources contribute to an "average internal heat gain" that is typically worth around 4.3°F. If you subtract the typical average internal heat gain from the typical building temperature (69.3°F - 4.3°F) you get a base temperature of 65.0°F. This is effectively the temperature that the heating system needs to heat the building to, as the average internal heat gain supplies the difference.
In the UK, for example, the majority of energy professionals primarily use degree days with a base temperature of 15.5°C. This is partly because 15.5°C base-temperature degree days are the historical norm in the UK, and partly because, unlike degree days with other base temperatures, 15.5°C figures are readily, and freely, available.
Different buildings are heated to different temperatures. Although it's often recommended that office buildings be heated to 66°F (19°C), in reality they are often several degrees warmer. Industrial buildings are often several degrees cooler. Average internal heat gain varies greatly from building to building. Clearly a crowded office packed with people and equipment will have a much higher average internal heat gain than a sparsely-filled office with a high ceiling. Clearly the internal heat gain from industrial processes depends greatly on the processes in question. In reality, 65°F or 15.5°C is rarely the most appropriate base temperature to use for degree-day-based calculations. This is important, as degree-day-based calculations can be greatly affected by the base temperature of the degree days used.