Source Energy

Source Energy

EPA(U.S. Environmental Protection Agency) has determined that source energy is the most equitable unit of evaluation. Source energy represents the total amount of raw fuel that is required to operate the building. It incorporates all transmission, delivery, and production losses. By taking all energy use into account, the score provides a complete assessment of energy efficiency in a building.

Commercial buildings use all types of energy, from electricity to natural gas to steam. To compare this diverse set of commercial buildings equitably, we must express the consumption of each type of energy in a single common unit.

We are familiar with site energy, which is the amount of heat and electricity consumed by a building as reflected in your utility bills. Looking at site energy can help us understand how the energy use for an individual building has changed over time. An image depicting the difference between the source and site energy.

Site energy may be delivered to a building in one of two forms: primary or secondary energy. Primary energy is the raw fuel that is burned to create heat and electricity, such as natural gas or fuel oil used in onsite generation. Secondary energy is the energy product (heat or electricity) created from a raw fuel, such as electricity purchased from the grid or heat received from a district steam system. A unit of primary and a unit of secondary energy consumed at the site are not directly comparable because one represents a raw fuel while the other represents a converted fuel.

Therefore, to assess the relative efficiencies of buildings with varying proportions of primary and secondary energy consumption, it is necessary to convert these two types of energy into equivalent units of raw fuel consumed to generate that one unit of energy consumed on-site. To achieve this equivalency, EPA uses source energy.

Using Median Site and Source Energy Use Intensity (EUI)

The national median source EUI is a recommended benchmark metric for all buildings. The median value is the middle of the national population – half of the buildings use more energy, half use less energy. The median works better than the mean (arithmetic average) for comparing relative energy performance because it more accurately reflects the mid-point of energy use for most property types and removes the effect of high-value outliers that may skew the data.

Sources:
Natural Resources Canada NRCan
https://www.energystar.gov/buildings/

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