We have a generally accepted way of measuring a building’s energy consumption: the EUI, or energy use intensity, a calculation of BTUs/square foot/year. This measure is becoming increasingly available to the public through energy benchmarking and disclosure requirements spreading rapidly across the country.
The rub is that the EUI, or an ENERGY STAR score derived from it, is not a measure of relative efficiency but only of whole-building consumption. It is a starting point, not a conclusion, about efficiency.
Industry professionals know that occupant density and specific use are among the variables that have enormous impact on a building’s energy consumption. The efficiency of the building systems is another matter and one that cannot be extrapolated from an ENERGY STAR score. But the general public may not recognize that.
An example is a New Republic article, “New York’s ‘Greenest’ Skyscraper Is Actually Its Biggest Energy Hog,” whose author concluded that One Bryant Park, a LEED Platinum office tower completed in 2009, was indeed the “hog” of the title. One piece of evidence offered by the article was that, according to New York City’s benchmarking data, the building consumed twice the energy per square foot as the much older Empire State Building.
The author’s comparison does not take into account the fact that the newer tower’s prime tenant is Bank of America, which houses its global corporate and investment banking operations there. In other words, the building has densely packed floors where workers hover over multiple monitors fed by high-performance computers and other hardware. (And one monitor alone can use more energy than a workstation computer.) The plug loads in such spaces are vast compared to many other office spaces. Moreover, the plug loads are a separate matter from the energy consumed by building systems.
The New Republic author also did not take into account One Bryant Park’s onsite generation capability via its 5.1-mW, low-emission cogeneration plant. Consumption data alone does not distinguish different sources of the electricity used in a building, just as it does not distinguish between plug loads and building system loads. Consequently, any energy from a building’s renewable sources as well as cogeneration would not be evident.
Energy benchmarking scores – whether good or bad – don’t provide the same basis of comparison as estimated mileage stickers on cars. Hopefully, the public will increase its awareness of the variables surrounding energy ratings and the comparative greenness of different buildings rather than become jaded about the ultimate value of energy efficiency in building design and operation.