Anatomy of an Energy Star Building Score
Most of us have heard of Energy Star certifications as ways to promote energy efficient products and buildings. The US Environmental Protection Agency (EPA) first created Energy Star as a voluntary program that could demonstrate potential for savings through decreased energy consumption. Today, the program has expanded across the United States and in to Canada. But calculation of building scores is not just a simple standard measurement across buildings. Let's go over how Energy Star scores are calculated for buildings, how they're normalized, and how this methodology differs among various groups of buildings.
What is an Energy Star Score?
The Energy Star Score, which ranges between 1 and 100, ranks an individual building’s energy efficiency on a percentile basis. In other words, if a building received a score of 75, then it would have been measured as performing more efficiently than 75 percent of its peers.
Introduced in 1999, the Energy Star building rating was created in the United States to rank peer buildings across categories like banks, hospitals, religious buildings, schools, offices, dormitories, supermarkets, and even warehouses. In July 2013, the EPA announced its collaboration with NRCan (Natural Resources Canada) to release a Canadian building score to compare peers in the same areas. Most buildings now have the ability to enter their information into Portfolio Manager, the EPA's free online tracking tool, to receive their scores.
5 Steps to Your Energy Star Building Score
The EPA has split the score process up into five overarching tasks to explain how the score is produced:
1. Enter building data into Portfolio Manager
The data must include 12 full months of complete energy use, specific building information, and details of energy use activity, such as average operational hours.
2. Compute the actual source energy use intensity
Actual source energy use intensity consists of the combined consumption of a building's use, including both the energy burned on site, as well as the energy being converted from its original source. To do this, the source energy consumption values are added across all fuel types, and divided by the gross floor area of the building. This final number represents the Actual Source Energy Use Intensity (EUI).
3. Compute the predicted source energy use intensity
Predicted source energy use intensity includes an average of the energy use intensity (EUI) based on the type of property, and makes adjustments based on working hours and number of people. These numbers are used to form a regression equation for each property type to determine the final predicted intensity.
4. Portfolio Manager computes energy efficiency ratio
Portfolio Manager creates an efficiency ratio, which represents your actual source EUI (from step 2) divided by the predicted source EUI (step 3). Lower ratios here indicate better performance.
5. Finally, Portfolio Manager uses the efficiency ratio in step 4 to assign a final Energy Star score.
Using a lookup table, higher percentiles mean more efficient buildings.
EPA Objectives for Successful Measurement
Evaluate energy performace for the whole building: Rather than evaluating specific devices that might be efficiently designed, but not correctly sized in relation to actual heating and cooling loads, successful measurement is aimed at whole-building performance.
Reflect actual metered energy use: These scores cannot be based solely on predicted energy use; they must include real use data for the most accurate numbers
Account for different energy sources: Using both source energy and site energy, combined totals are reflective of total costs and greenhouse gas emissions. Site energy includes the fuel used in building, while source energy is the generation and transmission of the fuel. Here's a helpful graphic from EPA to show how two buildings can produce the same heat load, but have differing efficiency levels based on the source energy.
- Normalize for building activities: Energy Star accounts for factors such as weather, operating hours, and number of workers in a building to get the most accurate comparison of peers.
- Provide a peer comparison: Much like we do here at WegoWise, these scores compare across similar peer groups with the same primary function, such as office space, school, etc. The percentile comparison includes survey sampling of more than 5,000 buildings across the U.S.
Buildings, Campuses, Mixed Use, and Multifamily
Campus scores are applied to properties with common campus areas, such as schools, hospitals, hotels, and senior centers. Instead of each building on the campus receiving its own score, these areas are assigned an overall score.
Building scores, on the other hand, are calculated only for individual buildings that do not exist in a campus setting. Most properties earn their own individual energy score, including situations like an office park that will earn building scores, but not an overall campus score.
Energy Star also accounts for buildings with multiple use spaces, like those with offices spaces in one area and hotel space in another. For these situations, each type gets its own regression equation. The prediction results are combined for one building score.
Energy Star scores are currently not available for multifamily buildings, but the EPA has recently announced that they are working on one that will be available in the summer of 2014 or later. They’ve built a model on data from 300 buildings with whole-building data, and will ask multifamily buildings to provide info such as square footage, number of bedrooms, and a zip code to produce a predicted energy use intensity score. Participating buildings are compared to this number to determine an Energy Star Score.
If you're interested in getting your data into Energy Star, WegoWise can help you automatically collect and send data from 180 different utilities to make it easy.