Heating and Cooling Degree Days 

In the United States, climate control is a major driver of energy consumption in the built environment. After all, it's a large country with a vast variety of different climates, ranging from below freezing temperatures in Alaska to the beachy paradise of Hawaii. It's no wonder people seek to make their homes as comfortable as possible. Especially when faced with extreme weather. So how do they do it? By using energy to heat or cool their homes. 

Living in the modern world can be very convenient when you have access to energy. Unfortunately, the process of making your space the perfect temperature can be a bit expensive. So the question becomes, how can people heat or cool their homes in the most affordable way possible? One effective way to estimate the energy required for climate control is through the calculation of heating degree-days (HDD) and cooling degree-days (CDD). These metrics provide valuable insights into how much energy will be needed to maintain comfortable indoor temperatures, helping to plan and optimize energy use in residential and commercial buildings alike. 

Understanding Degree-Days

So, what exactly is a degree day? A degree day is a compound metric that measures either the number of days with a 1° temperature change or the number of degrees changed within a single day, which can be useful for both heating and cooling needs. For instance, 10-degree days could represent ten days, each requiring a 1° adjustment or a single day needing a 10° adjustment. This metric helps quantify energy demand for heating (heating degree days, HDD) or cooling (cooling degree days, CDD) based on deviations from a base temperature. Homeowners can use degree day data to their advantage to estimate and budget for energy usage, optimize energy consumption with efficient systems, and save costs. Additionally, it assists farmers and gardeners in planning planting and harvesting schedules by providing a measure of heat accumulation essential for crop growth. Because of this, understanding degree days enables better management of energy needs, leading to cost savings and increased efficiency. Let's analyze these two measurements in more detail.

Heating Degree-Days (HDD)

Heating degree-days (HDDs) are calculated based on the difference between the daily average temperature and a base temperature of 65°F (18°C), which is considered a comfortable indoor temperature. If the average outdoor temperature is below 65°F, the difference between 65°F and the average temperature is the number of heating-degree days for that day. For example, if the daily average temperature is 55°F, that day accounts for 10 heating degree days (65°F - 55°F = 10 HDD).

Keep in mind that the number of heating degree-days varies by region and is influenced by local climate conditions. Colder regions accumulate more heating-degree days annually, indicating a higher demand for energy to maintain indoor warmth. On the other hand, milder climates accrue fewer heating-degree days.

Cooling Degree-Days (CDD)

Cooling degree days measure the energy required to cool a building. CDDs are calculated similarly to HDDs but in the opposite direction. When the daily average temperature exceeds the base temperature of 65°F, the difference between the average temperature and 65°F represents the cooling degree days for that day. For example, if the daily average temperature is 75°F, that day contributes 10 cooling degree-days (75°F - 65°F = 10 CDD). Regions with hotter climates accumulate more cooling-degree days, demonstrating a greater need for air conditioning to maintain comfortable indoor temperatures. Alternatively, cooler areas will have fewer cooling-degree days.

Heating and cooling degree-days are essential tools for estimating energy consumption in buildings. By analyzing the degree-days, energy providers and planners can predict the energy demand and optimize the supply accordingly. This information is also crucial for homeowners and businesses to understand their potential energy costs and to make informed decisions about energy efficiency improvements.

To delve deeper into energy concepts and explore a variety of engaging activities, visit the Watt Watchers of Texas. Read our latest blogs and discover fun energy games and activities that can empower you to become more energy-conscious and proactive in your daily life. Take the next step towards a more sustainable future with the Watt Watchers of Texas!

Activity 1: Looking at Data This activity is appropriate for individuals and for groups. Project or print the following map and graph for your students. Ask them the following questions about the data. U.S. Energy Information Administration Degree-day Regions * Texas is located in which region? [West South Central] * How many regions does the U.S. Energy Information Administration use to analyze degree-days? [9] * Which region has the highest number of heating degree-days? What is the number of heating degree-days in that region? [New England, about 6500 heating degree-days (6518)] * Which region has the highest number of cooling degree-days? What is the number of cooling degree-days in that region? [West South Central, about 2700 cooling degree-days (2747)] * Which region requires more cooling degree-days than heating degree-days? [West South Central] Activity 2: Calculating Degree-days Working as a class or individually, calculate the number of degree-days in a given week. If you undertake this activity in January, you may be considering heating degree-days. If you undertake this activity in August, you will likely be looking at cooling degree-days. The process is as follows.  Use the weather almanac, newspaper, or an online weather reporting service to collect and record the mean temperature for the day. Repeat this process for five days. Then calculate the number of degrees between 65°F and the mean temperature of each day. The spread of degrees on a single day is the number of degree-days to record for that day. If the mean temperature is higher than 65°F, then the day collects cooling degree-days, because cooling is required. The opposite is true for heating degree-days. If the mean temperature of the day is 65°F, then record 0 degree-days. At the end of the calculations, take a sum of the number of heating degree-days and the number of cooling degree-days. How many degree-days did your region collect this week? Each degree-day collected contributes to the consumption of energy for cooling and heating. The EIA uses a regional average to calculate the number of respective degree-days to assign to regions throughout the year, but this exercise will introduce students to how quickly energy can be consumed just to keep warm and cool.  Example Solution: The data in these tables was taken from a U.S. weather data almanac for Fredericksburg, Texas. The number of degree-days is calculated as the difference between the average temperature and 65°F. Negative differences denote heating degree-days, and positive differences denote cooling degree-days. Therefore, the dates 1 to 5 October accrue 26.15 cooling degree-days. The dates 27 to 31 October accrue 11.45 heating degree-days. A full explanation of degree-days and how the Energy Information Administration calculates and analyzes them is available through the Energy Units and Calculators Explained Series: Degree-Days.  These links are being provided as a convenience and for informational purposes only, and the inclusion thereof does not constitute an endorsement or recommendation. Although we make every effort to ensure these links are accurate, up to date and relevant, we are not responsible for content on third-party sites.