Energy use intensity (EUI) is one of the most important and useful metrics employed in the design of low energy and net zero energy buildings. EUI is a measure of total annual building energy use, divided by the gross building floor area.

This can be expressed in terms of site energy or source energy. Common EUI units include kBtu/ft2, kWh/ft2, kWh/m2, and MJ/m2. In the United States, kBtu/ft2 is the EUI unit most commonly used.

As we discussed in a recent article, most buildings in 2050 — less than forty years from now — will likely have to get by with perhaps 10% of the carbon footprint common in the U.S. today. We know of exactly one established building standard that's been demonstrated to produce 2050-ready homes, today. And, harsh though it might sound, anything built today that is significantly less efficient than the roughly 90% energy savings achieved by Passivhaus, seems designed to be obsolete. —The Editors