Ryan Pollin for Zondits, November 20, 2015. Image credit: Unsplash
It is difficult to look at a building from the outside and determine whether it is energy efficient. Dozens of window A/C units poking out would offer a big clue, but absent such evidence it’s not easy to know if a building uses lots or little energy. Even building age is an unreliable indicator! This makes building benchmarking all the more critical, since it’s our only way to “look under the hood” of a building, and to understand just what happens in that boiler room.
Let’s take a tour through a handful of perspectives in how to rank a building’s energy use.
Energy Use Intensity
Calculating a building’s energy use intensity (EUI), or kBtu/sq ft, is one of the most widely used metrics for understanding a building’s energy use. How much energy is used, and how big is the building? These are easy questions to ask—and easy questions to answer—but how much does this information tell you?
For example, is 100 kBtu/sq ft good or bad? Well, that depends on where on the globe you are located, what sort of work happens inside your building, how often the building is occupied… the list goes on. Your energy bills will indicate how much energy is actually used in your building (this is referred to as “site energy”), but to use the number as a benchmark against others’ usage, you might need to consider what form the energy takes and translate that to “source energy.” Source energy takes into account the origin of your district steam line, for example, and accounts for the energy back into the equation for fair comparison to buildings that make their steam from natural gas. This is commonly done with standardized conversion factors, ensuring that EUI is still a simple, hands-off metric.
This benchmarking metric is the easiest to calculate, and in return it tells you the least about how well a building is performing.
ENERGY STAR is likely the most widely recognized energy efficiency label in the country, having spent years on the labels of major household appliances. The US Environmental Protection Agency also runs a building-focused program called ENERGY STAR Portfolio Manager, which is a free service that tracks building performance. ENERGY STAR Buildings is one of the leading metrics in building benchmarking, and for good reason. Using 12 months of energy billing data and some basic background information, the program calculates a score of 1 to 100 for the building in question, with 50 being the 50th percentile of efficiency, 90 at the 90th percentile, etc.
The score tries to incorporate some of those open-ended questions that we couldn’t easily answer from just looking at kBtu/sq ft like weather and activity type. It slices up energy use by a number of variables and compares them to a standard data set called CBECS (Commercial Building Energy Consumption Survey). This US Energy Information Administration data set aims to represent the current building stock in the US. For many years, this data set had stagnated, stuck with the last update in 2003 – good cause for concern unless you wanted to know about historical performance from another age of building technologies. The good news is that new data based on 2012 consumption was released this past summer, which keeps ENERGY STAR at the top when it comes to understanding how well a commercial building is doing.
Determining this metric still takes just a modest effort and unlocks significantly more insight into a building’s performance.
Building Energy Asset Score
Following the ENERGY STAR’s program process, some questions remain. How much of your building’s rated performance was related your HVAC and lighting equipment, and how much of it was based on who was working and maintaining the building? Did you get a good ENERGY STAR score because everyone wore thick sweaters that winter instead of turning up the thermostat?
The Building Energy Asset Score (Asset Score) is a newer metric put out by the US Department of Energy, and it focuses on commercial buildings’ actual brick-and-mortar equipment performance. It shifts focus from the simple benchmarking question “How efficient is my building?” to a more sophisticated “How efficient can my building get?” For instance, how much better would your building score be if you invested in a free-cooling plant that cut out chiller loads during the winter? Or, the flip side: How well did your building maintenance staff do in keeping that building running as best as it could, and how much of their woes are actually related to dated equipment? The Asset Score teases out any effects that occupants or maintenance staff might have on a building. It normalizes in the case of tenants who have different preferences with the thermostat, etc., by looking only at the equipment in a building and that equipment’s potential for efficient operation.
A building’s Asset Score gives a more objective assessment of the building’s equipment, taking out effects like tenants and maintenance routines, rather than the building’s annual energy use, which is heavily dependent on the humans inside the building.
To balance out our focus on the commercial buildings, let’s give mention to the industry standard residential energy benchmark available – the HERS Index. HERS stands for Home Energy Rating System, and it looks at the materials and equipment in a residence and rates their performance. Unlike the others, the HERS Index is not based on actual energy consumption information. In this way, it has more in common with LEED New Construction certification than a benchmark: that is, it’s only a predictor. The rating is anchored at two points – an index of zero means that the home uses net zero energy – it generates as much as it uses over the course of a given year from some on-site energy generation. A HERS Index of 100 corresponds to, say, a newly built home that is as efficient as an industry standard practice construction, whereas the typical existing home gets an index of 130. A HERS Index is determined by a licensed auditor who will go on-site and inspect building materials in the envelope, look at the HVAC, lighting, and appliances throughout the building, and make some accounting for local weather. These inputs go through a program called REM/RATE, which predicts heating and cooling loads and spits out the HERS Index. The index is not a pure-play prediction of energy use, but its equation does reward for better building materials or ENERGY STAR appliances, etc., regardless of their direct energy savings in that installation.
The HERS Index splits the difference between describing the annual energy use and describing the building’s assets. It may not be perfect, but it’s the standard for benchmarking residential buildings.
The Zero Energy Performance Index, or zEPI, is a metric established by the New Buildings Institute, and it attempts to reframe building performance as it relates to energy codes and compliance. The zEPI is primarily an academic exercise thus far, and is fundamentally focused on shifting the conversation about energy codes. Energy codes create an artificial “minimum requirements” baseline for buildings to meet or exceed, and this is often the jumping point for creating incentives and other benefits. The ENERGY STAR score compares a building to the existing building stock, a general population of buildings of all ages and performances. zEPI suggests a third way, where a building is benchmarked on a theoretical 0-100 scale where 0 represents a zero net energy building (to anchor the other end of the scale, a 100 is defined as the average building in the CBECS 2003 database – or an average building’s performance in the year 2000. In some ways this is very similar to HERS, where the target value is a zero net energy building, but it applies to commercial and industrial buildings. What is most interesting is to rank code-efficient buildings on this overarching 0–100 scale, and watch energy codes ratchet down to be more and more strict. By New Buildings Institute’s measure, ASHRAE 90.1-2004 brought code-compliant performance down to a score of 75, where 90.1-2013 pulls it down to a 54. The most aggressive code that has been benchmarked thus far is the Boulder Energy Code 2014, which is equivalent on the score of a 41.
zEPI is a new construct that aims to reframe the conversation about energy codes with the long-term goal of a zero net energy building stock as the goal. So far, it is most interesting and useful on that larger-scale analysis of policies than as a building-to-building comparison.
A National Trend
As it turns out, building benchmarking is becoming a popular policy-directed approach to improving energy efficiency in many cities and states. In New York City, for example, buildings are required by Local Law 87 to perform routine building energy benchmarking and disclose results so that anyone can know how well their building performs. The figure above from the Institute for Market Transformation shows the recent wave of states and localities with policies that require some benchmarking and public disclosure. If you compare this effort to what fuel efficiency disclosure did for cars, it becomes clear that simply measuring and disclosing the performance data could be a big hit for our building stock. In fact, the simple act of benchmarking in these locales has been shown to save energy – 7% over three years. Some of this could be attributable to the Hawthorne Effect – behavior changes when people know that they are being observed. Of course, the real power of benchmarking is not to be satisfied with a building’s score, but to unlock some understanding of what can be done to improve the building, and many of these policies include provisions such that the benchmarking audit involves detailing improvement opportunities.
Energy benchmarking objectives are twofold, then. The most effective course of action is to spread some basic literacy about energy performance to building tenants and then inform the appropriate party that their boiler room could use some attention!