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Figure C1 below reports as index numbers over the period 1970 through 2011: 1) commercial building floor space, 2) energy use based on source energy consumption, 3) energy intensity, and 4) the year-to-year influence of weather.
- Activity: Since 1970, the quantity of commercial floor space has nearly doubled, with about half of that increase occurring after 1985. There was particularly rapid growth in floor space in the latter part of the 1990s, but Figure C1 clearly depicts some slowing after 2008 from weak economic conditions.
- Energy use: Commercial energy consumption, measured as source (or total) consumption (i.e., including electricity losses; see Terminology – definitions of energy), has increased in every year since 1985, with three exceptions, 2001, 2006, and 2009. The reductions in 2001 and 2009 were due to weak economic conditions compared to the previous year. The reduction in 2006 stemmed from warmer winter weather conditions. In 2011, total commercial energy consumption was about 53% higher than in 1985. Commercial sector energy consumption includes some non-building energy use, estimated to be less than 10% of the total. See Section A.2.1 of the 2014 comprehensive report for further discussion.
- Energy intensity index: Commercial building sector energy intensity grew only modestly from 1970 through 1985, but then increased almost 15% over the next 15 years. Beginning in 2000, there has been a sharp break in that trend, with the 2011 intensity roughly 6% lower than the 2000 level. Improvements in lighting and HVAC efficiency, controls, and more stringent building codes probably account for most of this improvement. Even after this most recent trend, commercial building sector energy intensity was about 8% higher in 2011 as compared to 1985 (this change in the "adjusted" source intensity does not include efficiency improvements in electricity generation that decrease generation and transmission losses over time; see Terminology – definitions of energy).
- Changes due to factors unrelated to efficiency improvements: Commercial buildings are sensitive to weather, although not to the same degree as residential buildings (generally influencing the index less than 1% from one year to the next). The chart below shows the estimated weather factor (as an index), based upon heating and cooling degree-days by census region. The years 1990 and 1992 stand out; for most of the U.S. calendar year 1990 contained very warm winter weather and 1992 was an especially cool summer. The decreases in intensity observed in 1991, the period 2001-2003, and in 2009 reflect weak economic conditions in the economy as a whole. In these periods, vacancy rates of commercial office and retail space increased and the utilization of occupied space fell.