Most people - especially those in the building operations industry - are familiar with the government’s Energy Star Rating (ESR) system. This 1 – 100 score helps building owners to understand how their building’s energy consumption measures up against other buildings nationwide. While ESR is an informative tool, the building block of ESR - Energy Use Intensity (EUI) - is the most important calculation when determining energy performance goals.
A building’s (or campus’s) current EUI can be quite easy to calculate, but a target EUI that is practical for your facility is much more difficult. If you’ve ever scratched your head at what EUI is, how to determine what your target EUI should be, or how to improve your EUI – here’s a quick summary.
WHAT IS ENERGY USE INTENSITY?
Energy Use Intensity (EUI) is a benchmarking metric used to express a building’s energy fitness. EUI – which has been called “the miles per gallon rating of the building industry” - is conveyed as a function of the building’s size and expressed as energy used per square foot per year. In the energy efficiency design world, EUI is the starting point in determining both baseline and target energy efficiency goals.
It’s important to understand that large variances in EUI are normal. When comparing one building’s EUI to another building’s EUI, differences in building function and climate is a major factor. In other words, some property types are more energy intensive than others. For example, an elementary school uses relatively little energy compared to a hospital; a building in a mild climate uses relatively less energy compared to a building in very cold or very hot climates.
There are two types of EUI – Site EUI and Source EUI. Site Energy refers to the amount of heat and electricity consumed by a building as reflected in a building’s utility bills. Source Energy is the total amount of raw fuel required to operate the building; basically, site energy plus all transmission, delivery, and production losses. Source EUI is what’s used when calculating Energy Star Rating.
HOW ARE EUI AND ESR RELATED?
ESR and EUI are interrelated, but there are differences to note:
- EUI is a Function of ESR: ESR calculations use the Source EUI of a building as an important input in determining the score. You can’t get ESR without EUI.
- A Low EUI Signifies Good Energy Performance: These metrics also differ in that a low EUI signifies good energy performance. This is the inverse of Energy Star Rating (ESR), where an ESR of 100 is considered the pinnacle of energy efficiency.
- ESR Makes it Easy to Compare Buildings of Different Function: Because ESR considers a variety of factors, including building type and climate, in its calculations, this score makes it easier to compare the energy fitness of two very different buildings. On the contrary, an EUI of 100 for an elementary school could be fairly high, for example, while 300 EUI in a hospital could be exceptional.
ENERGY STAR RATING LIMITATIONS
While ESR is a very useful tool, there are limitations as to when and how it can be used. In these situations, EUI alone can be used to determine baseline energy performance and assess targets.
- ESR is Only Available for Specific Building Types: One large limitation with ESR is that it can only be determined for buildings that fall under certain classifications. For a full list of ESR-eligible property types, check here: ESR Property Types
- ESR Must be Calculated on a per Building Basis: Want to determine the current Energy Star Rating of an entire campus? Not possible. Because ESR is calculated based off of building function, building scores must be calculated individually. For instance, a college campus with 12 separate buildings (offices, lecture halls, dorms), cannot receive one single Energy Star Rating. In this situation, focusing on EUI specifically is necessary. A single EUI baseline be determined for the entire campus and help set simple goals for the campus as whole.
HOW TO DETERMINE BASELINE EUI
Baseline EUI is calculated using the following formula.
EUI = Annual Energy Use (kBtus) / Building(s) Area (ft²)
HOW TO DETERMINE TARGET EUI
Determining the right target EUI for a building or campus is complicated, but imperative. This output varies greatly depending climate, building function, budget available, and most importantly the goals of stakeholders.
An EUI assessment – or the process of calculating baseline versus targets - is a great starting point. With just a few key pieces of information, we can help you to determine your baseline EUI, your target EUI and proven ways to reach your energy performance goals.