Energy Use Intensity (EUI) is a metric that expresses a building’s energy consumption relative to its size, typically measured in thousand British thermal units (kBtu) per square foot per year. It is calculated by dividing the total annual energy the building consumes by its gross floor area.
ANSI/ASHRAE Standard 100-2018 is another great reference. Addition infomation can also be found with this link: https://www.energystar.gov/buildings/benchmark/understand-metrics/what-eui?utm_source=chatgpt.com