ENERGY Glossary
EUI (kBtu/sf/yr)
pEUI (kBtu/sf/yr)
Explanation. Energy use intensity expresses a building’s energy use as a function of its size, typically in energy per square foot per year (kBtu/sf/yr). It’s calculated by dividing the total energy consumed by the building in one year (often measured in kBtu) by the total floor area of the building (often measured in square feet), and can be useful for comparing performance of buildings across sizes, types, and locations. When used before EUI, the letter "p" indicates that the data is predicted, based upon an energy model. The lack of a "p" indicates actual measured EUI.
Examples. Energy intensive homes and buildings might have an EUI between 100 and 200 kBtu/sf/yr, while high performance homes and buildings might have an EUI of 25 kBtu/sf/yr or less. The Passive House standard requires less than 14.6 kBtu/sf/yr.
Air Leakage (ACH50)
Explanation. Air changes per hour (ACH) is a measure of the air leakage of a building, calculated using a blower door at a standard pressure difference of 50 pascals (a unit of pressure) between inside the home and outside. A building's ACH50 therefore tells us that at the pressure of 50 pascals, air leaks from the building at a rate of X times the volume of the building per hour.
Examples. A leaky building might be 15 ACH50, while a high performance building is typically 1 ACH50 or less. The Passive House standard requires 0.6 ACH50 or less.
Explanation. A home energy rating is an analysis of a home’s energy efficiency; as per the Home Energy Rating System (HERS) Index. The HERS Index is a scoring system for measuring a home’s energy performance. The HERS Index Score can be described as a sort of miles-per-gallon (MPG) sticker for houses, providing some insight as to how the home ranks in terms of energy efficiency.
Examples. Existing homes often have a HERS Index between 120 and 200, while a new home built to code would be a 100, an efficient home might be a 50, a net zero energy home would be a 0, and a net positive home would be less than zero.
HERS
Explanation. The building energy code in a given area sets the minimum legal limit for performance of a building. One way to express increased energy performance is to compare the building to the minimum legal limit.
Example. If a building constructed to code used 50 kBtu/sf/yr and a high efficiency building just met the Passive House standard and used 14.6 kBtu/sf/yr, then the high efficiency building could be considered to perform 70% better than code.