Defining Energy Use

In the table on trends in the story on sustainable buildings, "Energy Performance Data Largely Lacking," residential energy consumption should be calculated in millions of Btu per household per year, and not kBtu (thousands of Btu) per household per year. It is more definitive to look at the year of construction and then the energy consumption in Btu per sq ft per year.

For those houses built in the 1970s, the consumption was 45,500 Btu per sq ft per year, while those built in the 1980s used 41,400 and those built in the 1990s used 38,500.

Similarly, commercial building energy consumption should be expressed in thousands of Btu per sq ft per year, not millions or hundreds of millions. In addition, to lump all types of commercial buildings together is not meaningful.  The data from the Dept. of Energy's Energy Information Administration Commercial Building Energy Consumption Survey, which is conducted every four years, is thus comparable.

What it shows is that the average for all commercial buildings from 1992 to 2003 has been 80,000 to 91,000 Btu per sq ft per year. This is nowhere near the more than 200 million Btu per sq ft per year shown in the trends table. The table shows that some commercial building types, such as food and health-care, do use in the neighborhood of 200,000 Btu per sq ft per year, whereas education, mercantile and office buildings use less than 100,000 Btu per sq ft per year.

As further evidence, 293 kWh of electricity at 10¢ per kWh, or $29.30, represents 1 million Btu, so a building using 200 million Btu per sq ft per year would have an electric bill of $5,860 per sq ft per year–not a realistic value. If a building used 200,000 Btu per sq ft per year, the electric cost would be $5.68 per sq ft per year. In reality, newer office buildings typically use from 50,000 to 100,000 Btu per sq ft per year, which equates to an electric bill of $1.42 to $2.84 per sq ft per year, which is more in line with what your readers experience and expect.

Therefore, both the residential - and commercial-building energy intensities in the trends table are wrong. Residential intensities are 80 million to 100 million Btu per household per year, not twice as much, and commercial intensities are  from 40 thousand to 250 thousand Btu per sq ft per year, not hundreds of million Btu per sq ft per year, depending on building type. Much more extensive CBECS data are available at http://www.eia.doe.gov/emeu/cbecs/contents.html.

 Larry Spielvogel, P.E.
L.G. Spielvogel Inc.
King of Prussia, Pa.