No one enjoys paying higher home heating bills during an unusually cold winter or spending the extra money on increased electric bills due to the hot summer months. As expensive as monthly energy costs are for homeowners, however, they generally represent a more significant expenditure for businesses with large facilities.
If you’re like most people, you spend little time thinking about heating degree-days. In fact, unless you’re a building or plant manager in charge of energy consumption, you may not have even heard of heating degree-days. But having a basic understanding of the degree-day concept will assist you in knowing what factors affect energy consumption for your home or small business and may eventually lead to cost savings.
What Are Heating Degree-Days?
At one time or another, you’ve probably taken advantage of a relatively pleasant day by opening your home’s windows and enjoying the comfortable weather. On days like these, you turn off your heating or cooling system — or more likely, your thermostat mechanism keeps your air conditioner or heater from turning on unnecessarily.
On the other hand, when the weather is consistently cold, your heating system works nearly around the clock to keep your house at a cozy average temperature. The same thing happens when your air conditioning runs continuously during the summer. In both scenarios, your energy bills are higher because of the more frequent, if not continuous use, of your AC or heater.
The degree-day data concept establishes a standard to account for fluctuating temperatures. The data can be used to predict heating and cooling trends as well as to determine whether energy consumption is the result of weather fluctuations or caused by other factors, like faulty equipment or breaches in the structure of a building. Heating degree-days and cooling degree-days are similar concepts. We will examine both in this article, but for now, the focus will be on heating degree-days (HDD).
A degree-day is defined as the number of degrees the average temperature of a given day is above or below the temperature a building should be heated to or cooled to for comfort. The standard temperature number is 65o Fahrenheit (18o Celsius).
If the temperature 65 degrees Fahrenheit seems a little on the cool side to you, keep in mind that most buildings have occupants, appliances, office machinery and equipment that all generate heat. This is referred to as internal heat gain, and it makes the interiors of a building warmer.
Even with those heat gains, though, if outside temperatures drop below 65 degrees, it’s going to start getting uncomfortably cold inside the building unless you use your building’s heating system. Because temperatures fluctuate throughout the day, the best models use multiple temperature readings throughout the day and calculate an average.
Calculating Heating Degree-Days
The best way to understand a heating degree-day is to see how it’s calculated. The formula is as follows:
(65oF – average temperature in oF) x 1 day = number of heating degree-days
For example, if the average temperature was 63 degrees Fahrenheit on a particular day, it would represent two heating degree-days:
(65oF - 63oF) x 1 day = 2 HDD
If on the next day the average temperature fell to 61 degrees Fahrenheit, that day would represent four heating degree-days.
(65oF - 61oF) x 1 day = 4 HDD
If the temperature is 65 degrees Fahrenheit or greater, it’s calculated as zero heating degree-days. Negative numbers aren’t used. So, if the average temperature was 66 degrees Fahrenheit on a particular day, the calculation might look like this:
(65oF - 66oF) x 1 day = -1 HDD
But that wouldn’t be right. Again degree-days must be positive numbers, so this wouldn’t be added as a heating degree-day.
So, let’s use our calculations to represent a three-day period:
As you can see, the 66-degree day didn’t affect the total. The total number of heating degree-days for the three-day period is six. Fortunately, there are sites that collect and crunch these numbers, so you don’t have to keep a spreadsheet and plug in daily numbers to benefit from the data.
How Is the Average Temperature Calculated?
While the calculations may be simple — even simpler with a computer — finding an average temperature presents some difficulties. If on a particular day the temperature were to hold steady at 64 degrees Fahrenheit, the average temperature calculation would be easy. In more typical weather patterns, however, the temperature changes continuously. The more temperature measurements taken on any particular day, the more accurate the reading will be for calculating the heat degree-day measurement for that day.
Most people wouldn’t be too enthused about spending their day checking thermometers to take multiple temperature readings, and fortunately, they don’t have to.
Websites like DegreeDays.net will help you identify a weather station near you, select a base temperature — it defaults to 65 degrees Fahrenheit (18 degrees Celsius), and provide you with your heating degree-day calculation.
What Is Degree-Day Data Used For?
If you’ve read through the explanations and were able to follow how degree-day data is calculated, you’re probably also interested in its purpose. Climatologists and other scientists who require accurate temperature-trend data for projects that can be affected by weather find degree-day data useful to determine whether a given year was an anomaly or part of an overall trend.
Businesses in the agriculture industry — one of the areas of the economy most affected by shifting weather patterns — require degree data to determine expenses associated with irrigating, growing and harvesting crops. Farmers and horticulturists use growing degree-day data to determine growth cycles of animals and plants Commodities traders look at heating day-data to try to forecast anticipated demand for energy sources like heating oil and natural gas.
For a business or homeowner, heating or cooling degree-days can be used to measure the efficiencies of their heating and cooling systems or other cost-saving measures they’ve instituted to lower power bills.
For example, let’s consider the management team of a Philadelphia-based factory who is trying to determine why their heating bill in December was higher than the previous year. Was it a cold winter? Was it due to heat leaching from the building? Was the gas heater working less efficiently — that is, consumes more gas to get the same job done? Was it a combination of the three?
Before the company goes out and starts pricing new equipment or contracting a firm to upgrade their insulation, they will probably want to know if a colder winter could have accounted for the increased heating bills. It’s also the fastest and least expensive factor to examine.
For this example, we used the weather data from Philadelphia Wings Airport. We would generally select a weather station close to the business, so in this example, the business is located close to the airport. We set the baseline temperature to the default of 65 degrees Fahrenheit and utilize the calculator on DegreesDays.net website.
The information provided by the site indicates the following:
- December 2013 – 849 HDD
- December 2014 – 792 HDD
- December 2015 – 506 HDD
Unfortunately for the business management, the increased heating bills can’t be explained away by colder temperatures. In fact, December 2015 was a much warmer month than the previous two years. This would indicate the need to look at other possibilities for their increased energy consumption.
There could, of course, be an even easier explanation. Utility companies sometimes change their pricing. A higher electric, gas or heating oil bill can just be the result of a rate hike or a change in pricing structure.
Can Homeowners Use Degree-Day Data to Lower Their Bills?
Even if you aren’t the manager of a large multi-facility business or a commodities trader, you can still benefit from degree-day data to examine the efficiency of your cooling or heating system.
According to a U.S. Energy Information Administration Report, households in Pennsylvania consume 96 million Btu per year, which is 8% higher than the U.S. average. This results in higher costs for these residents as well, about 16% above the national average.
The report goes on to say that approximately half of that energy is from space heating. Conversely, only 3% is used on air-condition space cooling. The takeaway for Pennsylvania residents is that if they’re looking to save money on energy bills, winter heating is the place to start.
There are several ways to lower your heating bill, including:
- Upgrading to an energy-efficient heating unit
- Adding additional insulation to your attic to prevent heat escaping from your residence
- Performing an integrity check on your home’s structure for cracks and fissures
- Replacing weather stripping around doors and windows
- Keeping window treatments open to allow in sunlight
But as we’ve previously discussed, a higher gas or electric bill may not be an indication there’s anything wrong with your heating system or the seals around your windows and doors. By doing a few minutes of research, you can look up the historical heating degree-days data from a weather station close to your home and determine weather or not you’re paying more because the winter is colder.
How Do I Use Heating or Cooling Degree-Days?
Collecting and analyzing heating degree-data is relatively simple:
- Locate a weather station near your home – Bizzee Degree Days and Weather Underground both give you access to free weather station data. If you live in a metropolitan area, you will probably have several weather stations near your home. For all intents and purposes, the data from any close weather station is sufficient to identify weather trends, but if you insist on having the absolute closest location, you can put the addresses into Google maps to calculate the distance and choose the closest one.
- Choose heating or cooling degree-days – People in northern states, like Pennsylvania and Maryland, may only focus on heating degree-days, while residents in Arizona and New Mexico may find only find cooling degree-day data useful. More on cooling degree-days in the next section.
- Choose a base temperature – Some degree-day sites allow you to select a base temperature other than the standard 65 degrees Fahrenheit. If, for instance, you feel that your house is still too cold when you set the thermostat to 65 degrees, you may want to use 68 degrees as your base temperature. Naturally, the higher the base temperature, the higher the number of degree-days the calculator will generate. An increase in the base of one degree Fahrenheit automatically adds one degree-day for the day calculated.
- Select a period for measurements – You’ll need to determine both the intervals you want to examine and the time period you’re interested in examining. For example, because energy bills are sent on a monthly basis, to compare your past and current heating bills, you may want to look at monthly heating degree-days data over the past three years.
- Compare the numbers – The numbers the calculator generates should give you a pretty clear picture. For example, if you experience a December with 250 heating degree-days one year and 300 HDD’s in December of the next year, with all other things being equal, you’d expect a 20% increase in your bill. All other things are rarely equal, but for the purpose of this example, we will assume you have not upgraded your heating system, changed your home’s insulation or increased your consumption for non-heating purposes.
We want to emphasize that degree-days don’t always tell the full picture. It’s easy to explain away bill hikes by pointing to an increase in heating degree-days, but the fact that they play a significant role in energy consumption is irrefutable.
What’s the Difference Between Heating and Cooling Degree-Days?
As you may have noticed, the emphasis has been on heating degree-days. That’s because the markets we serve at Smart Touch Energy are disproportionately impacted by the cold. Some U.S. climates rarely get colder than 65 degrees Fahrenheit, however, so they have zero heating degree-days. But degree-day data can be just as useful in warmer climates, and they are referred to as cooling degree-days.
To find the number of cooling degree-days, the first part of the formula is reversed to reflect temperatures above the 65-degree threshold:
(Average temperature in oF – 65 oF) x 1 day = Number of cooling degree days
If the average temperature is 70 degrees Fahrenheit, the number of cooling degree-days is five. As was the case with heating degree-days, there are no negative cooling degree-days. It’s either a positive value, or the day has no degree-day value.
Commercial vs. Residential Use
Although few homeowners are even aware of degree-day data, it’s essential information for those involved in energy budgeting in industrial operations, large office buildings, hospitals and governmental facilities. Some businesses find degree-day data so essential to the functioning of their business that they purchase their own weather-station equipment to continuously provide on-site temperatures.
Degree-day data assists us in anticipating our commercial and residential customers’ demands, and enables us to create beneficial pricing schedules. The Energy Information Administration even uses population-weighted degree-days to help predict energy consumption for various regions of the country or for the United States as a whole. The calculations are weighted by the population of a region according to census data.
While heating degree-days haven’t actually become a household word, it may interest you to know that Pennsylvania is one of the ten most popular states for degree-day data downloads. Philadelphia is one of the ten U.S. cities that download the most degree-day data. Word must be catching on.