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What is EUI?
Energy use intensity (EUI) is an indicator of the energy efficiency of a building’s design and/or operations. EUI can be thought of as the miles per gallon rating of the building industry. It is used in a number of different ways including to set a target for energy performance before beginning design, to benchmark a building’s designed or operational performance against others of the same building type, or to evaluate compliance against energy code requirements. It is important to remember that EUI varies with building type[i]
. A hospital or laboratory will have a higher EUI than a residence or small office building.
EUI is expressed as energy per square foot or meter per year. It is calculated by dividing the total energy consumed by the building in one year by the total gross floor area of the building. EUI is expressed as thousands of British thermal units used per square foot per year (kBtu/sq. ft./year) or gigajoules per square meter per year (GJ/m2/year). To calculate EUI, energy used for one year must be converted from kilowatt hours of electricity or therms of natural gas to kBtu or GJ.
What is Energy and how does it affect EUI?
The amount of energy used by a building can be considered in two different ways, at the building site or at the source, meaning where it was generated[ii]
. It’s important to know which definition of energy you’re using. There is also a third definition, Time Dependent Valuation (TDV) energy, used only in California in the Performance Approach to documenting compliance with the California Energy Commission’s Title 24 Part 6 Building Energy Standards.[iii]
Site energy is the amount of energy consumed at the building site indicated in utility statements or via metering for an existing building or as predicted by energy modeling software for a building in design. Utility delivered energy plus Renewable Energy generated and used onsite are included because they are required to operate the building. Renewable energy exported to the electric grid is not included because it is not used for building operations. Site EUI is the amount of site energy used in one year divided by the total square feet of building area.
Source energy traces heat and electricity used at the site back to the original raw inputs. Electricity, for example, can be generated at a power plant by burning raw fuels such as coal or natural gas, from clean sources such as large hydropower plants, or from renewable ‘fuels’ such as sun, wind, small hydropower, and geothermal. Source energy includes the total amount of raw fuel used at power plants to operate a building.
In calculating source energy, a ‘multiplier’ or ‘factor’ is applied to metered site energy to reflect the raw fuel used to generate and deliver the metered site energy. The Source Energy Factor used by the US EPA as of 2019 for utility delivered electricity was 2.8.[iv]
As grid-supplied electricity replaces fossil fuels with renewable sources such as sun and wind, this multiplier grows smaller.[v]
In calculating source energy, the amount of utility-supplied electricity used on site would be multiplied by the Source Energy Factor. Onsite solar or wind energy used in building operations has a Source Energy Factor of 1.0, meaning there is no multiplier. For natural gas used at the building, the Source Energy Factor is 1.05. Adding together utility delivered energy with the multiplier applied, plus the actual amount of on-site renewable energy used in operations gives the total amount of source energy required to operate the building. Renewable energy exported to the electric grid is not included because it is not used for building operations. Source EUI is the total amount of source energy used in one year divided by the square feet of building area.
Net EUI adds consideration of the amount of renewable energy exported to the grid in a year. The amount exported is subtracted from Annual Energy Use and the remainder is divided by square feet of building area. A Net Zero Energy building has a Net EUI of zero.
How can I use EUI in architectural practice?
Before starting design, you can determine the Site Energy Use Intensity (EUI) baseline for the building type and set a target Site EUI for your building.
During design you can use energy modeling to compare design alternatives. At key points in the process you can use the model to estimate the amount of energy your proposed building design will and then calculate Site EUI. In this way you’ll learn whether you are on track to meet your energy target and revise the design if necessary.
You can sign on to the AIA 2030 Commitment and use EUI to document the energy performance of your projects. Through the 2030 Commitment you can evaluate portfolio wide energy performance of your firm’s projects and see where your Firm stands in relation to others in your region and nationwide.
You can estimate performance of past projects by using spreadsheets available from AIA California to derive Site EUI from 2013 and 2016 Title 24 Part 6 whole building compliance reports.[vi]
What should I explore and do next?
1. Benchmarking and EUI by building type.
· Energy Star provides median Source and Site EUI numbers for many building types here:
· Architecture 2030 uses benchmarks as a starting point in establishing energy performance targets for design. See: https://zerotool.org/about/
2. Setting energy targets for your projects before beginning design.
· The Architecture 2030 Zero Tool can guide you in setting energy targets for both EUI and percent reduction compared to a building type average from the 2005 Commercial Building Energy Consumption Survey. See:
Median EUI for different building types in the USA can be found at https://www.energystar.gov/buildings/facility-owners-and-managers/existing-buildings/use-portfolio-manager/understand-metrics/what-energy
The concept behind TDV is that energy efficiency measures that save energy should be valued differently depending on which hours of the year the savings occur, to better reflect the actual costs of energy to consumers, to the utility system, and to society. TDV multipliers are established for each hour of the year for electricity and month of the year for gas. They are applied to predicted energy use within Title 24 compliance software. TDV EUI is the total amount of TDV energy used in one year divided by the square feet of building area. Because it is California specific, TDV EUI cannot be used with national energy benchmarking standards.
The EPA does not report Source Energy Factor for subsectors of the national electric grid. However, as of 2018 the carbon intensity of the CAMX grid serving most of California was 496.5 lbs/MWh or 52% of the 947.2 lbs/MWh number for the national grid as a whole. [https://www.epa.gov/energy/power-profiler#/
] This reflects the high percentage of California’s electricity generated from renewable energy sources and “clean” large hydropower. Nuclear power supplied 9.1% of California’s electricity in 2018. California’s sole remaining nuclear plant, the Diablo Canyon Nuclear Power Plant, will close in 2025 and will be replaced by sources and grid management strategies that are greenhouse gas free. Source Energy Factor and carbon intensity of the electric grid are different metrics. However, lower carbon intensity of the electric grid does suggest that electric powered equipment on the CAMX grid requires less greenhouse gas emitting source energy than in other regions of the USA.