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Figure R1 below reports as index numbers over the period 1970 through 2011: 1) the number of U.S. households, 2) the average size of those housing units, 3) residential source energy consumption, 4) energy intensity, and 5) an overall structural component that represents "other explanatory factors."
- Activity: Since 1970, the number of household (occupied housing units) has nearly doubled, increasing from 63 million to over 114 million in 2011. Compared to 1985, the number of occupied units grew by 30%. The average size per housing unit grew modestly between 1970 and 1985, but has increased by about 25% since 1985. Using the estimate of average floor area per occupied housing unit, total residential square footage has more than doubled since 1970 and increased by over 60% since 1985.
- Energy use: Residential energy consumption, measured as source energy (i.e., including electricity losses) increased by about 17% between 1970 and 1985, and by 33% since 1985. In terms of actual energy use, source energy in 1985 was 16.0 QBtu, and by 2011 had grown to 21.4 QBtu.
- Energy intensity index: The energy intensity index, based upon energy use per square foot, has declined in nearly every year since 1970. The greatest annual percentage declines occurred in the 1970s in response to the sharp increases in energy prices that accompanied the oil supply shocks. Since 1985, the greatest declines were observed in the early part of the 1990s and in the mid-2000s. Since 1985, the intensity index has declined by 15%. (The source intensity index presented here has been adjusted to remove the effect of increased efficiency in the electricity generation sector – an element that reduces the generation and transmission losses over time. For comparison, without that adjustment and using the conventional measure of source energy to compute the index, the reduction since 1985 would be 19%. See Energy Intensity Changes by Sector, 1985-2011.)
- Changes due to factors unrelated to efficiency improvements: As shown in Figure R1, consumption declined in 1990, 1997, 1998, 2001 and 2006, years of mild winter weather in much of the U.S. The effect of weather from one year to the next has contributed to annual changes in the intensity index as large as 3% to 4%.
Aside from weather, there are several other explanatory factors that account for energy use that are unrelated to the efficiency of energy use by households: shifts in the distribution of housing units among regions and among housing types (from single-family homes to condominiums, for example). The structural index (see discussion in Efficiency vs. Intensity) shown above is a composite of these shifts that also includes the effect of weather, which has a much greater short-term influence on energy consumption than do shifts in the number of households by region or in the types of housing. The regional and housing-type shifts together account for less than a 1% decline in energy consumption over the 1985-2011 timeframe.
Another perspective on residential energy intensity considers the trend in source energy use per household (or occupied housing unit), rather than per square foot as shown above. Figure R2 shows a historical series that also adjusts the source energy to exclude efficiency improvements in the electricity sector. Had this utility improvement been included, the 2011 value of the index would have been about 4% lower than that shown in the figure below (compare to Figure R1 where the conventional measure of source energy implicitly does include this improvement). This alternative measure of residential energy intensity shows about a 7% increase between 1985 and 2011, although there has been distinct downward trend in the index beginning in 2006.