Michael R. Davis: What’s the Best Way to Measure Energy Use?

Michael R. Davis

Net-zero-energy building is first and foremost about radical energy-use efficiency.

Commercial, academic, and multi-family residential buildings use lots of energy. The National Renewable Energy Laboratory, a division of the U.S. Department of Energy, estimates that the average existing commercial building uses 90 kBTUs/SF annually in all forms of energy combined.

kBTUs/SF? Okay, a little refresher on units of measure. 

When discussing a building’s energy use, what unit of measure should we use? How about “watts per square foot”? That’s generally what we use when we’re talking lighting load. It’s good because it contains that “per square foot” part, so we can use it in design criteria. But it only counts electricity (watts) and it’s a static measure of the allowable maximum power of a lighting system. It doesn’t really say anything about actual use. 

A  familiar term is “kilowatt-hours.” That’s a gross measurement of electric power actually consumed. A utility uses this unit when preparing your bill. But to use it as a design standard, first you need a time period (a month or a year) and then you do the math and divide over floor area. And again, it’s electricity only.

Then there are “therms” of natural gas and gallons of fuel oil. These involve the BTU, or British Thermal Unit, which is unit of the energy content of any kind of power. When used in the design of buildings or spaces, this unit lets us lump all power sources together. When stipulated as “per square foot” and “per year”, the humble BTU—or kilo-BTU—can be used to describe a building’s energy needs per unit of floor area per time period. This is the metric we should be using.

Now, back to the National Renewable Energy Laboratory. Researchers there say that the typical commercial building burns 90 kBTU’s per square foot per year. Given the state of technology available today, for this building to become net-zero, it would have to burn no more than 30 kBTU’s per square foot per year, as a rule of thumb. That’s a two-thirds reduction. 

But when you think about it, what would be the point of loading an inefficient building with photovoltaics or wind turbines to try to make it net-zero? It’s pointless unless efficiency is addressed first. Besides, you’d need a couple football-field’s worth of photovoltaics to meet the power needs of a bare-bones, 20-year-old, three-story suburban office building. Okay, that was a wild guess. But net-zero-energy building is not possible without radical energy efficiency first.

Radical? Our commercial building designers are benchmarking new projects at about 50 kBTU/SF per year already, and they think the high-30s are achievable without rocket-science technology. In fact, new building standards are already driving down buildings’ energy use. —Michael R. Davis