What are Degree Days?

If you live in the northern part of the U.S. and heat a building by natural gas, the gas bill from the utility will often report a number called “heating degree days.”

Why is this number useful? “Heating degree days” is related to the average outside air temperature for the billing period.   It makes sense that the colder it is, the more energy your building uses to keep you warm.   A particularly mild winter?   You use less energy for heating.   If you are trying to understand whether changes in energy use come from changes in outside air temperature, your operations or equipment changes, or some combination, you need to know about outside air temperature.   And of course, as an Energy Steward you want to assess whether you are making progress in using energy more intelligently, no matter what the outside air temperature!

Do you want the science, with some formulas, that connects heating a building and temperature, expressed in degree days?   Take a look at David Mackay’s explanation in technical Chapter E of Sustainable Energy: Without the Hot Air, the book described in this recent post.

Even without the science, you can see how degree days can help us understand building performance.  Let’s start with the arithmetic for “heating degree days”:

1. Pick a reference temperature (the typical temperature is 65 degrees F in the U.S.  We’ll use this number).

2. For each day in a time period, take the difference between 65 and the average daily temperature.

Example:  On March 14 in Madison, WI, the average daily temperature was 53 degrees.  So the heating degree days associated with March 14 is 65-53 = 12 heating degree days.

What if the temperature for a day on average is above 65?  For a simple heating system in a simple building, you won’t need to heat the building much.   So people agree that the heating degree day should be zero any time the average daily temperature is 65 or above.

Do you want the heating degree days for the 33 days of a utility bill?  You need to know the average daily temperature for each day, compared to 65 degrees, and then add those differences up to get a total for the month.

Many utilities save you the trouble of doing this arithmetic by giving you the total heating degree days on the bill.   This is a nice service, since many utility bills don’t match exact calendar month periods and the number of days in the billing cycle may change, month to month.

With heating degree days and energy use, what’s the connection?  In many buildings, there is often a surprising, simple relationship.

Here’s an example for a 134,000 square foot middle school in southern Wisconsin.  We used data from the utility from January 2010 through April 2012.  Wow!  We see roughly a straight line relationship between therms and HDD!

So what?   The simple relationship means that you can predict energy use—for example, for a time period with 1000 HDD, we predict the building will use a bit more than 6000 therms, perhaps about 6300 therms.   Of course, the prediction depends on the future being about the same as the past, in terms of hours of operation, the patterns of equipment use, and the mechanical details of the heating system.  Let’s call this past set of conditions the “baseline conditions.”   If you change the school to use energy more efficiently, you can compare the predicted use given baseline conditions to the actual use and our comparison has accounted for changes in outside air temperature.    If the actual value is less than the predicted value by more than the scatter of the baseline points about the line, that’s a pretty good sign that, accounting for weather, energy use is less.

Extensions

  1. If you know how to get an equation from a plot using a spreadsheet or other tool, you can plug in different HDD values to get different predicted therms.   You can fit relationships more complicated than a straight line, too.   The equation that matches the straight line shown in the middle school example is Therms = 2.69 x Heating Degree Days.
  2. Buildings also use energy to cool them off, so you shouldn’t be surprised that people have invented “cooling degree days.”     You still need a reference temperature but now the arithmetic focuses on temperatures above the reference temperature.    Example:  On August 20 in Madison, WI, the average daily temperature was 77 degrees.  What are the cooling degree days based on a 65 deg F reference associated with August 20?  12: because 77 – 65 = 12
  3. ENERGY STAR uses degree day information but looks at temperature totals 12 months at a time in reporting ENERGY STAR ratings and weather adjusted energy intensity numbers.   We like monthly data to get a basis for simple monthly predictions.
  4. If you have daily meter readings, you can use the same approach:  now plot daily energy use vs daily HDD (or equivalently, the daily outside air temperature.)   You can get daily temperature from a building control system equipped with outside air sensor or from a nearby weather station  www.weather.gov/climate/  is the portal to access reports from NOAA weather stations.  There are also database services that for a modest fee will provide you tables with climate information for thousands of locations around the country.
  5. Engineers use the same ideas as I’ve described above to model energy use in buildings.   They may start with engineering formulas (again, check out the MacKay book, chapter E) or use more factors than HDD to predict energy use.   In fact, ENERGY STAR calculates weather adjusted energy use by the same approach we sketched.
  6. In Energy Stewards®, you have the option to enter heating degree days and cooling degree days when entering meter records.   These values are stored and available for analysis when you download records as a CSV file on your detailed energy use page.
This entry was posted in Useful Ideas. Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *