Degree Days for Dummies
May 12, 2016

There are many complicated charts, graphs and calculations that experienced energy professionals use for weather normalizing usage data for various reasons.  In our industry, this is most important when determining contracted volumes for natural gas futures contracts. There are software programs that are specifically designed to process these high-level calculations with the end-goal being to identify signs of waste and asses recent energy performance while allowing for weather correction.

The layman can use the following basic calculations to achieve (hopefully) the same end result – although it is not recommended:

  1. Gather historical consumption. Assemble 12 months of bills or request a usage history from your utility.
  2. Determine your baseload volumes. Baseload volumes represent steady natural gas usage independent from processes and equipment used for heating. An easy, but not too accurate way to figure baseload volumes is to take the 3 months of the year with the lowest usage and average them.
  3. Determine heat therms. The total therms minus the baseload are the heat therms.
  4. Get updated Heating Degree Days (HDD). You can pull them by week or month depending on how your meter is read.  Using the right weather station is important.
  5. Figure your Heating Energy Intensity (HEI). This is the heat therms divided by the HDD, divided by the conditional square footage of the area.

Use the HEI to compare one building/facility in a portfolio against another, or the national average for like building types/uses. The HEI can also be used year-over-year to evaluate the implementation of energy efficiency measures like boiler replacement and lighting retrofits.

The results are a lot less accurate than people realize.  Failure to figure the precise baseload volumes, choosing the wrong weather station and/or utilizing incorrect formulas can lead to erroneous outputs and further frustration.

Before paying for a software program to do this for you, consider Energy Star Portfolio Manager. Portfolio Manager provides for a similar gauge called Energy Use Intensity (EUI); which removes transmission and distribution losses before calculating raw fuel consumption against square footage and then compares this number against like buildings. You can also input past, present and future projects and establish baselines on which to approve.

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