by Tristan Roberts. — With both greenhouse gas emissions and the cost of energy escalating, there is more focus than ever on understanding just how much energy our buildings use, and then figuring out how to use less. In this article we’ll review the key ideas around measuring energy use, and then look at major programs for both homes and commercial buildings that provide deeper analysis.
KEY ENERGY METRICS: Site, Source, Cost & Carbon
There are several ways that we gather the raw data on energy use in buildings, discussed here. In addition, there are numerous composite metrics that create ratings from this raw data. Those are discussed below in a separate section.
Site Energy
Site energy measures the amount of energy consumed at a facility—a number that is reflected on gas and electric utility meters, and on meters attached to any onsite renewable sources. Site energy is measured in terms of whatever energy inputs are being used at a building. To find a single number summing up these inputs, it’s necessary to convert them to a single measure—in the U.S., usually million Btus (see the table for selected energy conversion factors). What’s a “good” site energy figure? The table Energy Targets: Site and Source Averages offers some points for comparison. Advantages of the site energy metric are that it is easy to measure and comprehend, and that it puts the focus of efficiency at the location where building owners and operators have the most control: the building. It incentivizes efficiency at the building level and gives full credit to onsite renewables in offsetting energy from utilities. However, site energy does not present the total picture of the energy used in extraction, production, transmission, or conversion of the energy delivered by the utility—expenditures that are reflected in the source energy measure.
Source energy
Source energy includes site energy consumed at the building, and also includes energy used offsite to generate and transport the energy that is used at the building. In Europe, source energy is often called “primary energy.”
In the U.S., on average and according to figures from Energy Star, it takes 3.34 units of energy for a power plant to deliver 1 unit of electricity and 1.047 units of fuel to deliver 1 unit of natural gas. These measures, and similar measures for other energy sources, are the source-site ratios that reveal the total energy expenditures behind that site energy. To calculate a building’s total source energy, both energy brought onto the site and energy that leaves the site (from onsite photovoltaics, for example) are multiplied by the appropriate source-energy factor (see table).
Advantages of the source energy metric are that it provides a full accounting of “hidden” costs of energy consumed onsite. Most notably, electricity can be an incredibly efficient energy source within buildings, but source energy reveals the inefficiencies in delivering that energy. A disadvantage of source energy is that it is not an obvious concept for most people, and it requires additional research and time to calculate….
Energy cost
Energy cost simply measures what a building operator is paying for energy. This definition uses market valuation to account for the relative value of various fuels.
One benefit of the use of energy cost as a metric is that it puts energy use in simple terms that anyone can understand, and relates directly to where most people feel the pain of energy use (and the pleasure of savings)—in the pocketbook. Energy cost also reflects, to an extent, the difference between site and source energy. Although market prices are constantly changing, electricity is relatively expensive compared with fossil fuels, including natural gas. Source energy metrics tell us that that’s how it should be, since electricity is a more refined fuel compared with natural gas, and has higher per-unit source energy. In practice things get a little more complicated, though (don’t they always?), because non-residential users also pay demand charges, which are fees assessed based not on how much energy you use, but how fast you use it when you’re using it fastest. Also, in California and other places, electricity prices vary by time of day or by demand on the grid.
Carbon
Ultimately the reason many people are focused on energy use in buildings is because of the associated carbon emissions. Each of the metrics described so far is related to carbon emissions, but none tells the whole story—that can get a lot more complicated, and depend on the exact fuel mix at any point in time. In fact, it’s not just the fuel mix at the building that matters, but also the fuel mix at the power plants that send electricity to the building—and those mixes are constantly changing. As a result, most carbon footprint estimates rely on average values for carbon intensity per kilowatt hour—either national averages, regional averages, or averages for a specific utility company. These averages, and even power-plant specific data, are available from the U.S. Environmental Protection Agency’s EGrid database.
More details on how these numbers relate, and the implications of choosing each approach, are laid out in “Counting Carbon: Understanding Carbon Footprints of Buildings” on BuildingGreen.com.
Energy use intensity (EUI)
Both site and source energy are usually normalized by building size. We calculate energy use intensity (EUI) for a building by taking the total energy consumed in one year (usually measured in thousand Btu, or kBtu) and dividing it by the total floor space of the building. For example, if a 50,000-square-foot school consumed 7,500,000 kBtu of energy last year, its EUI would be 150 kBtu/ft2. When looking at EUI numbers, check whether they are “site EUI” or “source EUI”—both are common. Source EUI is favored by Energy Star Portfolio Manager, but site EUI is more often cited by campaigns such as the 2030 Challenge.
Normalizing by building area may be the most common method, but it is not without limitations. Buildings with more occupants per square foot, or more operating hours relative to their peers, can be penalized. Even measuring the area of a building is not necessarily straightforward. Realtors distinguish between gross square feet and rentable square feet; energy modelers care whether the space is conditioned, unconditioned, or semi-conditioned—and we’re just getting started. EUI can also be normalized by weather.
Variations in source energy
It’s convenient to use a single national average for site-source energy ratios, but it doesn’t tell the whole story. With all energy sources, but particularly with electricity, the source energy expenditure varies by region, season, and time of day.
Time-of-day variations result from fluctuations in demand, and from resulting changes in how utilities supply in power. Peak electricity demand usually occurs around mid-day during the summer as most businesses and factories are in full operation and as cooling demand peaks. Utilities bring on additional generation at these times to meet demand, and the cost of that energy in terms of source energy may be higher. At night or in the winter, higher-efficiency plants provide the “base load” power for the grid, and the source energy expenditure for electricity is lower.
California’s Title 24 energy code puts a focus on this issue through what it calls TDV, or time-dependent valuation of energy use. TDV assigns fluctuating dollar values to electricity, gas, and propane based on the time of day and time of year that they are consumed. By requiring buildings to be more efficient relative to TDV, and not a more static measure of energy use, California encourages buildings to not only reduce overall demand but to also reduce peak demand.
Region-by-region differences in source energy and carbon intensity of electricity are another area of debate. For example, three-quarters of power generation in Washington State is from hydropower. Should consumers in Washington be able to use a more favorable source-to-site energy ratio?
Conclusion
It would be nice if there were a clear answer to the question of which metric to use, but it just ain’t so. A useful case in point is the new Bank of America tower at 1 Bryant Park in New York City—it focused on energy cost and source energy use in achieving a LEED Core and Shell Platinum rating. But on the metric that most designers use, site EUI, it doesn’t look so good, according to this exposé. At the end of the day, you have to understand them all, and guide your work based on the one that’s most meaningful to you.