it over time, and the timeframes in which specific technologies can be expected to penetrate the market, the committee was able to develop illustrative scenarios of how total energy consumption could evolve. Confronted with myriad, diverse manufacturing industries, the committee focused on the five most energy-intensive industries. The committee examined other technologies, although in less detail.
For each sector, comparisons were made to a baseline, or business-as-usual, case in order to derive the potential for energy savings. For the buildings and industrial sectors, this was the reference-case scenario of the U.S. Energy Information Administration’s (EIA’s) Annual Energy Outlook 2007 or 2008 (EIA, 2007a, 2008161). For the transportation sector, a committee-directed baseline was derived. In all cases, though, the study estimates the level of energy-efficiency improvement beyond the baseline or reference case. More details can be found in the report titled Real Prospects for Energy Efficiency in the United States (NAS-NAE-NRC, 2009).
In 2006, the United States had approximately 81 million single-family homes, 25 million multifamily housing units, 7 million mobile homes, and 75 billion square feet of floor space contained within 5 million commercial buildings (EIA, 2008). The building stock is long-lived; homes last 100 years or more, commercial buildings often last 50 years or more, and appliances used in buildings last 10 to 20 years. In 2008, residential and commercial buildings accounted for 73 percent of total electricity use in the United States and 40 percent of total primary energy use (Figures 4.1 and 4.2).
Use of delivered energy in the residential sector increased by 15 percent from 1975 to 2005, and in the commercial sector it grew by 50 percent. Meanwhile, primary energy grew by 46 percent and 90 percent, respectively, in the residential and commercial sectors. Despite these increases, energy “intensity”—energy use per unit of service or activity—decreased over that time span.
In the residential sector, on-site energy intensity, measured as energy use per household, fell by about 33 percent during 1978–2001, while primary energy use per household declined by 20 percent. In the commercial sector, on-site energy intensity, measured as energy use per square foot of floor area, dropped by about