Energy Use Intensity (EUI) is calculated by dividing the total energy consumed by the building in one year (measured in kBtu) by the total gross floor area of the building.

To understand EUI you need to note that there are two definitions of EUI.  EUI can take into account the losses from converting raw energy sources into consumable energy at the building level with a site or source distinction. Losses from energy generation, transportation, and so forth are the primary difference between site and source EUI.

The simpler of the two, site EUI, is calculated with the total energy use of the building at the meter, regardless of the source.  Natural gas, electricity, and solar PV are all converted to one unit (kBtu) and counted equally.  A source EUI, on the other hand, takes into account the total upstream energy the building consumes, which means it’s always larger or equal to the site EUI.

The exact difference between the site and source EUI is a function of the fuel mix of the building, as well as the building’s location. Different regions of the country have varying energy sources. Typically, source to site is a factor of 2-3 to 1. You can use examples of national fuel source conversion factor from Energy Star Portfolio Manager.

Strategies for achieving energy targets are provided in the Advanced Energy Design Guide for Zero Energy for K-12 School Buildings which discusses how to set measurable goals and offers example EUI targets for your climate zone. The EUI’s provided in both site and source, represent the technical feasibility targets for ZE in different climate zones from a study done by ASHRAE, NREL and the Department of Energy.  It is important note that these are generally about new construction, but can be applied to ZE school retrofits as well.

It is recommended that ZE school targets should be set to a site EUI of 25 kBtu/sf/year or LESS! This varies dependent on climate zone but you will notice that even in the most difficult climates, the site EUI still is at 25 kBtu/sf/yr or less.