In the absence of a standard or benchmark it is difficult to compare the energy uses between buildings or structures. Simply measuring the amount of energy used per a chosen time period does not take into account building size, configuration or type of use. The use of an Energy Use Intensity (EUI) indicator provides the means to equalize the way that energy use is compared between various types of buildings, and evaluate the means of reducing overall energy consumption.
When using EUI, energy use is expressed as a function of a building’s total area. In the United States, EUI is typically expressed in energy used per gross square foot of building per year. It is calculated by dividing the total gross energy consumed in a one-year period (expressed in kilowatt-hours or kilo-British Thermal Units) by the total gross square footage of the building. For example:
Calculating a Building's EUI
A school contains a main floor consisting of 15,000 square feet, a second floor consisting of 10,500 square feet. The school used 1,170,000 kilowatt-hours of power during the year in question. Kilowatt-hours is multiplied by 3.412 to obtain kBTUs, therefore 1,170,000 * 3.412 = 3,992,040 kBTUs. This is divided by the total square footage of 25,500 square feet for an Energy Use Intensity of 3,992,040 / 25,500 = 156.55 kBTU/sf/year.
What Affects a Building's EUI?
EUI can vary significantly depending on building type. Hospitals have EUIs that can range from 400 to 500 kBTU/sf/year, due the high energy demand of interior lighting and hospital equipment. In contrast, a school may have an EUI in the range of 150 kBTU/sf/year. Food services facilities tend to have very high energy usage, and can have EUIs above 800 kBTU/sf/year. EnergyStar has a list of building types with median site and source EUIs.
Climate can have a significant effect on EUI, due to the variations in heating and cooling costs between different areas of the country. For this reason, EUI values may be broken up into region to provide a more accurate comparison of selected structures, or the values may be “weather-normalized” to adjust the EUI to be compared against a building in a different type of climate.
The US Energy Information Administration (EIA) compiles information via the Commercial Buildings Energy Consumption Survey (CBECS) that allow for a comparison of energy consumption based on building sizes and types of use. EIA has set energy use reduction challenge targets for the year 2030 that comprise energy use reductions of 70%, regardless of building type or use.
Reduction of EUI
Some of the methods used to reduce energy use intensity are:
- ensuring proper maintenance of equipment to improve efficiency of operation
- installing motion activated lights (occupancy sensors)
- incorporate the use of natural sunlight into the design of occupied spaces
- provide a means for passive heating and cooling of interior spaces
- develop on-site renewable energy generation
Heating, air conditioning, and lighting in building spaces together comprise the majority of energy use and obtaining efficiencies in these two areas can result in a significant amount of cost savings, as well as gains in compliance with the 2030 energy reduction goals.