Measuring Energy Use in Buildings: Do Our Metrics Really Add Up?

BuildingGreen.com
May 23, 2011

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

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....