Understanding Source and Site Energy

EPA's national energy performance ratings evaluate the performance of buildings that use all types of energy. To compare this diverse set of commercial buildings equitably, the ratings must express the consumption of each type of energy in a single common unit. EPA 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, thereby enabling a complete assessment of energy efficiency in a building.

• The difference between site and source energy
• The use of national conversion factors
• Technical methodology in Portfolio Manager

The difference between site and source energy

Most building managers are familiar with site energy, the amount of heat and electricity consumed by a building as reflected in utility bills. Site energy may be delivered to a facility in one of two forms: primary and/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, in order 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 the convention of source energy.

When primary energy is consumed on site, the conversion to source energy must account for losses that are incurred in the storage, transport and delivery of fuel to the building. When secondary energy is consumed on site, the conversion must account for losses incurred in the production, transmission, and delivery to the site. The factors used to restate primary and secondary energy in terms of the total equivalent source energy units are called the source-site ratios.

The use of national conversion factors

The efficiency of secondary energy (e.g. electricity) production depends on the types of primary fuels that are being consumed and the specific equipment that is used. These characteristics are unique to specific power plants and differ across regions of the country. For example, some states have a higher percentage of hydroelectric power, while others consume greater quantities of coal. Because ENERGY STAR is a national program for protecting the environment through energy efficiency, EPA has determined that it is most equitable to employ national-level source-site ratios. As such, there is only one source-site ratio for each of the primary and secondary fuels in Portfolio Manager, including electricity. The use of national source-site ratios ensures that no specific building will be credited (or penalized) for the relative efficiency of its utility provider.

Technical methodology in Portfolio Manager

For more detailed information on source energy, refer to Energy Star Performance Ratings: Methodology for Incorporating Source Energy Use (435KB). This complete technical document provides detail on the distinction between site and source energy the value of performing source energy comparisons. In addition, the document provides details on the policies for incorporating renewable energy, the philosophy behind the use of national factors, and the specific calculations used to derive each conversion factor.