Hugh Crowther
2021-02-19 07:27

Buildings use more energy than any other sector so it stands to reason that measuring energy use in buildings is a key performance indicator.

An Energy Use Intensity or EUI is measure of how much energy a building uses per area over a given period of time.  Most commonly it is measured as kBtu/ft²-yr or kW/m²-yr.  A Zero Energy Building or ZEB is a building with an EUI = 0.  Building use and location are the two most critical influencers. Building use takes into account hours of operation while location takes into account the climate.

Site vs. Source Energy

It is critical to consider where the energy consumption is being measured.  Site Energy measures the energy consumption as the energy crosses the property line.  The site energy EUI can be calculated by looking at the energy consumption on the property energy bills (electricity, gas, central plant etc) for the year and dividing it by the building area.   Site energy is straightforward to calculate and closely connected to the energy operating  cost of the building but it does not accurately reflect the total energy consumed to operate the building. 

Source Energy considers how much energy was consumed to deliver usable energy to the building.  For example source energy would take into account how much natural gas was used to run the turbine that produced the electricity the building consumed.   

Depending on whether you are considering  Site or Source energy, the building EUI will change.  The difference can be significant.  For example the site energy uses for a hospital is 234.3 kBtu/ft²-yr while the source EUI is 426.9 kBtu/ft²-yr.

Converting from Site to Source energy requires a factor that is typically supplied by a government agency such as EPA or DOE. 

CO₂e Emission Factors

The Table above shows emission factors from ASHRAE Standard 189.1 – Standard for the Design of High-Performance Green Buildings. It is based on US national averages.

EUIs and Building Performance Benchmarking

Looking at the building’s EUI based on real energy use gives a good understanding if the building is improving or degrading over time.  It is good way to see if operational changes or building upgrades have been effective.

Building energy performance benchmarking lets the building owner understand how their building compares with other similar buildings in the same climate zone.  This is what ASHRAE’s Building EQ and the EPA Portfolio Manager (Energy Star) programs do.  They analyze the building EUI and compare it to a database of building types by climate zone.  Building benchmarking is a very powerful tool for improving energy performance of the existing building stock.

Most of the databases used by benchmarking tools come from the Commercial Building Energy Consumption Survey or CBECS.  Every few years, the USDOE sponsors a survey of real buildings covering a wide range of building types and climate zones.  CBECS is used to evaluate the real world improvement from changes to standards like ASHRAE Standard 90.1  as well as being the backbone for benchmarking.  It is also used to calibrate national building energy use models to  help identify critical areas of improvement and future energy needs.

Median EUI Tables

To see the Energy Star Portfolio median EUIs for US building types click here

To see the Energy Star Portfolio median EUIs for Canadian building types click here