ENERGY STAR Ratings: The numbers behind the 1-100 scale

We recently posted a description of the ways Energy Stewards® uses the ENERGY STAR® ratings.   In this post, we’ll give a bit of background on the 1-100 scale–where it comes from and how it gets updated.

The 1-100 scale is a yardstick developed from a surveyed set of facilities of a specific type.

Each 1-100 yardstick is FIXED and does not vary by the number of buildings tracked in Portfolio Manager.

Most* of the 15 types of facilities for which there is a rating use the Commercial Building Energy Consumption Survey (CBECS) managed by the federal Energy Information Agency.  The most recent CBECS survey used by ENERGY STAR is based on 2003 data.

Usually, CBECS is conducted every four years but a technical glitch caused the 2007 survey to be set aside; the 2011 survey got caught in budget cuts.   The good news is that there is a survey planned that will use 2012 data, to be conducted in early 2013.   The data will then be available to ENERGY STAR next year to redo the older yardsticks.

For each building type, ENERGY STAR creates a separate yardstick.   EPA staff and consultants look at energy use for each building.  They also look at a set of factors that influence energy use like weather, size and additional building attributes, e.g. hours of operation and numbers of PCs for office buildings.  The technical wizards at EPA take all this information and create a statistical model that expresses the energy in terms of the factors.  Next, they rank the buildings in the survey from lowest energy use to highest.    Finally, they map those ranks into a 1-100 scale in a sensible way.  For example, the 50 point rating value on the scale maps to the middle (median) of the set of ranked buildings.

If you’re interested in the technical details, this link describes the general method.   The ENERGY STAR site also has a separate document with statistical details for each building type; the documents are hard to find.   The easiest way we know is to enter “Technical methodology” in the ENERGY STAR page search box.   You will then find a series a pdf files with address of the form http://www.energystar.gov/ia/business/evaluate_performance/XXX, where XXX will be a distinct pdf file name for each of the 15 building types.

As the yardsticks age, they become less useful for making statements about the absolute performance of a building, relative to all buildings of its type.  Building standards, equipment, and practices do change in important ways over 10+ years.

Older yardsticks still are useful for tracking relative progress over time and comparing batches of buildings because the yardsticks themselves don’t vary.

So, keep using the ENERGY STAR ratings in your energy management program and look for several revised yardsticks sometime late in 2013!

(*A few of the scales use special surveys and have newer yardsticks.  Scales for hospitals, senior care facilities, data centers and waste-water treatment plants were developed over the last couple of years  through partnerships between EPA and interested third parties like the American Society of Healthcare Engineering (ASHE).)

 

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