What are degree days?

The degree days are a rationalized form of historical weather data. The degree-day data is simple and easy to work with. Degree days come in daily, weekly, and monthly breakdowns. They are usually used in energy monitoring and targeting, to model the relationship between energy consumption and outside air temperature.

The two major types of degree days are heating degree days (HDD) and cooling degree days (CDD). Both of these types can be celsius or fahrenheit based. 1 therm to be calculated is equivalent to 1,00,00 British Thermal Units (BTU). Degree days are essentially a simplification of historical weather data and to be precise, they are the outside-air-temperature data. 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 or hourly temperature readings.

Heating degree days (HDD)

Heating degree days (HDD) are used in the calculations that are concerning to the heating of buildings. For instance, HDD may be utilized to standardize the consumption of energy of buildings with central heating.

They come with a "base temperature", and give a measure of how much temperature (in degrees), and for what time period (in days), the outside temperature was below that base temperature. In the UK, heating degree days have come across a base temperature of 15.5°C whereas, in the US, it's 65°F. Though, these days it is simpler to get HDD at any base temperature.

For instance, if the outside temperature was 2 degrees for 2 days under the base temperature, there shall be a total of 4 heating degree days over that time period (2 degrees x 2 days = 4-degree days). In real life, the process of calculating accurate degree days is cumbersome. So, the calculation of the degree days must be done by an individual himself.

Cooling degree days (CDD)

Cooling degree days (CDD) are used in the calculations that are concerning to the cooling of buildings. For example, cooling degree days can be used to standardize the energy consumption of buildings with air conditioning. Cooling degree day figures are inclusive with a base temperature and produce a measure of the rise (in degrees), and for what time period, the outside temperature was over that base temperature.

Celsius or Fahrenheit

Celsius-based degree days are calculated by utilizing the base temperature and the exterior temperatures metered in celsius.  Fahrenheit-based degree days are calculated using a base temperature and the outside temperature measured in fahrenheit. They both contain their calculations.

Weather normalization

Application of the degree days is the weather normalization of energy consumption. It enables the differentiation of energy consumption from various periods or distinct places with discrete weather conditions. Weather normalization is solely for energy consumption that is influenced by the weather, which means escalating heating and cooling. As there is a lot of energy usage, there is additional information to energy-data analysis than the weather-related techniques. Since heating or cooling energy consumption is fundamental in nearly all buildings, its analysis is without any doubt important.

Weather-normalization techniques are many times based around regression analysis of past energy-consumption data method that is regularly used with degree days to:

• Demonstrate or calculate energy savings
• Keep track of the energy usage in progress for any waste indication (excess consumption) and check the ongoing progress for reducing it. The constant tracking and monitoring help to distinguish between the recent consumption with a past-performance-based estimate of anticipated consumption (explicitly based on a regression model).

The underlying matter is that the heating/cooling energy consumption of buildings is complex. In heated or cooled structures, energy consumption depends on the outside air temperature. Weather correction permits regulation of energy consumption figures, to consider the changes in outside air temperature. Weather normalization is generally required to analyze any alterations in the energy consumption of the building.

In the relevant historical weather data (mostly HDD), evaluation of weather-normalized energy consumption in 2020 as well as the weather-normalized energy consumption in 2019 is feasible and can be differentiated for the enhancement of the energy efficiency of the building.

Using degree days in energy monitoring and targeting

Simple ratio-based weather normalization of energy consumption

Heating degree days are often used to standardize the energy consumption of a heated building so, heating degree days would make an individual capable to calculate normalized energy consumption entries. The easiest path to normalize energy-consumption entries is to calculate the degree day kWh per each kWh energy-consumption entry. In numerical charts, utilize the 2019-degree days as the multiplier to leave behind the 2019 energy usage unaltered and to normalize the 2020 energy usage to 2019 weather conditions.

Regression analysis of energy consumption

Regression analysis is thoroughly used in energy monitoring and targeting. For a heated building, it is predicted that the energy consumption required to heat the building for a given time frame is driven by the number of heating degree days over that time frame. Plotting of the monthly degree days (x-axis) against the monthly energy consumption (y-axis), evaluation of the monthly baseload energy consumption can be done from the place at which the regression line crosses over the y-axis.

The relative effect of base temperature on the degree days

The degree day’s base temperature may have a huge effect on the proportional separation between the degree days of one month and the next month. If weather-normalizing monthly energy consumption details (weekly or daily) are computed, the incorrect choice of base temperature can easily give misleading results. To further increase the complexity, the base temperature of various buildings changes slightly throughout the year. The clarified model of degree days (and base temperature) is not the representation in real life. It is vital to choose a correct base temperature for degree day analysis, and the most correct base temperature is not likely to be "default" in the specific country. As the building's base temperature usually changes slightly all around the year, even the most proper base temperature is typically only an approximation.

Problems with general degree-day-based methods

The baseload-energy problem

The concept of baseload energy is significant but it can be tedious to calculate the baseload energy precisely. Regression analysis is one method to calculate the baseload energy. To show the base temperature effect on the calculated baseload energy, care must be taken that degree days never correlate perfectly in reality.

The effect of degree-day base temperature on evaluation of baseload energy

The HDD in all three base temperatures correlates systematically with the energy consumption and all the R2 values are at their peak.

The ideal-temperature problem

Degree-day analysis is typically less accurate because the flaws in approximation become more apparent on days. Receiving a great regression model is of vital importance for almost every degree-day-analysis process, such as the one suggested for calculating energy savings. The degree days.net regression tool makes it user-friendly to get a great baseline regression model. Also, Interval metering helps further, if numerous interval meters are installed. 

Utilize degree days effectively

• Give preference to regression-based methods to simple ratio-based methods
• Work out the best energy data
• Use the most suitable degree-day data
• Select a yearly timescale for weather-normalized data comparisons
• Be particularly cynical about the normalized data for time frames with an ideal outside temperature
• Keep in mind the precision level
• Take a look at the proportional differences before the absolute numbers

Context and Applications

• Bachelors of Technology (Electrical Engineering)
• Masters in Science (Energy Conservation and Sustainability)
• Masters in Science (Power Consumption and Management)

Practice Problems

Q1. One therm equals how many British Thermal Units?

1. 1,00,000
2. 100
3. 10
4. 1

Correct Option- a

Explanation: One therm equals 1,00,000 British Thermal Units.

Q2. In the US, heating degree days have come across a base temperature of how much °F?

1. 35
2. 45
3. 55
4. 65

Correct Option-b

Explanation: In the US, heating degree days have come across a base temperature of 65 °F.

Q3. Which one of the following is/are the type(s) of degree days?

1. Heating degree days (HDD)
2. Cooling degree days (CDD)
3. All of these
4. None of these

Correct Option- c

Explanation: The types of degree days are heating degree days (HDD) and cooling degree days (CDD)

Q4. Which of the following statement is incorrect?

1. Work out the best energy data
2. Use the least suitable degree-day data
3. Keep in mind the accuracy level
4. Select a yearly timescale for weather-normalized data comparisons

Correct Option- b

Explanation: The most suitable degree-day data is to be used.

Q5. What is the unit of electrical energy consumption?

1. Kg
2. kWh
3. N
4. J

Correct Option-b

Explanation: The unit of electrical energy consumption is kWh.

Related Concepts

• Heat, ventilation, and air conditioning (HVAC)
• Insulation
• Electrical Systems