Portfolio Manager Technical Reference: Source Energy

Commercial buildings all use different mixes of energy including electricity, natural gas, fuel oil, district steam, and many others. In order to evaluate energy performance for these buildings, we have to express all of these different energy types in a single common unit. Source energy is the most equitable unit of evaluation, and enables a complete assessment of energy efficiency.

You may be 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 energy is the raw fuel that is burned to create heat and electricity, such as natural gas or fuel oil. Secondary energy is the energy product created from a raw fuel, such as electricity purchased from the grid or heat received from a district steam system. A unit of primary energy 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. Ultimately, buildings require heat and electricity to operate, and there are always losses associated with generating and delivering this heat and electricity. Source energy traces the heat and electricity requirements of the building back to the raw fuel input, thereby accounting for any losses and enabling a complete thermodynamic assessment.

Figure 1, below, illustrates two buildings, which are identical in their construction and operation and require 100 MBtu of steam for heating. Building A purchases natural gas from a utility to produce steam onsite, whereas Building B purchases steam directly from a utility. That is, Building A is purchasing primary energy while Building B is purchasing secondary energy, and both buildings provide the same amount of heat to meet the demands of the occupants.

Figure 1 Two Identical Buildings Heated by Steam