Measuring energy: site energy vs. source energy in ENERGY STAR Portfolio Manager

To benchmark effectively, all different energy types must be expressed in a single common unit. Source energy is the most equitable unit of evaluation, which enables a complete assessment of building-level energy efficiency.

The Canadian adaptation of ENERGY STAR Portfolio Manager includes a conversion from site energy to source energy.

Site energy vs. source energy: what’s the difference?

Site energy accounts only for the energy used directly by your property, as reflected on your utility bills. It is considered to be the simplest way to measure a building’s energy consumption. However, site energy does not account for energy losses incurred during the production, transmission and delivery of energy to your property. Also, it is unable to properly account for different fuel mixes that may include primary energy sources (raw fuel burned on-site to create heat or electricity, e.g. natural gas or fuel oil) and/or secondary energy sources (energy created elsewhere, e.g. electricity purchased from the grid). This makes it difficult to accurately compare the energy use of two buildings that use different types of energy. A primary unit of consumption and a secondary unit of consumption on-site are therefore not directly comparable, one representing a raw fuel and the other a transformed product.

Source energy, on the other hand, accounts for all the energy used to power your building. It is essentially a conversion representing a combination of primary energy and secondary energy into a single common unit. It therefore includes the losses incurred during production, storage, transmission and delivery. This provides a more complete picture of your building’s energy requirements and enables a more equitable comparison of building energy use, neither crediting nor penalizing a building for the type of fuel it uses.

Frequently asked questions about source energy

Although the use of source energy has been selected and tested to ensure that it offers an accurate and equitable assessment of a building’s energy use, you may have some questions about how it accounts for various types of energy. Read on to learn more.

• Is source energy a fair and accurate comparison metric when compared to site energy?
• How does the use of source energy provide a fair assessment of a building’s performance, independent of the types of energy being used?
• Does the use of source energy affect a building’s ability to get a good 1-100 ENERGY STAR score?
• Does the use of source energy affect the ability of buildings that rely exclusively on electricity to get a good 1-100 ENERGY STAR score?
• Does greener energy mean a better 1-100 ENERGY STAR score?

Is source energy a fair and accurate comparison metric when compared to site energy?

Source energy:

• Allows for a whole-building assessment that combines all fuels
• Evaluates all buildings fairly, regardless of heating system
• Fairly evaluates electric heating in relation to natural gas and steam systems
• Provides equitable comparison of steam systems with natural gas-fired systems
• Fairly compares natural gas boilers with different on-site efficiency levels

Site energy, although the simplest to understand, has a number of limitations that make it unsuitable as a metric for nationwide comparisons of similar buildings:

• Site energy does not account for all the possible fuel mixes used by buildings across the country, because they are expressed in a variety of units that are not directly comparable.
• Site energy does not account for all the energy required to power a building, including generation and transmission losses, which must be taken into account for a full and accurate picture of a building’s energy use.
• Site energy unfairly credits the use of purchased secondary energy. See How does the use of source energy provide a fair assessment of a building’s performance, independent of the types of energy being used? for more details.

How does the use of source energy provide a fair assessment of a building’s performance, independent of the types of energy being used?

Source energy was selected as the basis for comparison in Portfolio Manager specifically because it does not provide either a credit or a penalty to buildings using any particular type of energy. In fact, the reverse is true, and the use of site energy would effectively credit buildings that purchase energy produced off-site by a utility. For buildings that purchase raw fuel (such as natural gas) and convert it to power on-site, a site energy measurement would account for the losses that occur during this conversion, because site energy is based on the fuel that is purchased. For buildings that purchase energy (such as electricity) generated off-site, a site energy measurement would not account for those losses, because they occurred before the energy reached the building. Thus a building generating its own power from a raw fuel source would appear to use more fuel to produce the same amount of energy as that purchased by the building whose energy is generated off-site by a utility provider. In both cases, there were losses sustained in the generation of the energy, but by using a site energy measurement, the building that purchases energy generated off-site would not be accountable for those losses.

Does the use of source energy affect a building’s ability to get a good 1-100 ENERGY STAR score?

Although a source energy measurement is usually higher than a site energy measurement, this does not affect the ability to achieve a high 1-100 ENERGY STAR score, because the methodology and calculations used to determine the score are also based on source energy. All buildings are assessed using the same criteria, so the use of source energy has no impact on a building’s ability to earn a high score.

Does the use of source energy affect the ability of buildings that rely exclusively on electricity to get a good 1-100 ENERGY STAR score?

Many buildings that rely exclusively on electricity for their power and heat have achieved 1-100 ENERGY STAR scores of at least 75 (representing the top-performing 25 percent of similar buildings). In fact, the U.S. Environmental Protection Agency found that 26 percent of ENERGY STAR-certified buildings (those with a score of 75 or higher) in the United States use electricity exclusively. This is only slightly lower than the 30 percent of all commercial buildings nationwide that are 100 percent electric. Furthermore, the same study showed that the average proportion of electricity in all commercial buildings in the United States is 62 percent, while the average among ENERGY STAR-certified buildings is 78 percent.^{Footnote 1} Taken together, these findings demonstrate that a high proportion of electricity usage does not prevent a building from earning a 1-100 ENERGY STAR score of 75 or higher.

Does greener energy mean a better 1-100 ENERGY STAR score?

Not necessarily. Portfolio Manager offers some credit for green energy generated onsite (e.g. through solar panels or wind turbines). Green or low-impact energy generated offsite and purchased from a utility may lower your carbon emissions but does not have a direct impact on your score.

Portfolio Manager’s focus is energy efficiency. As such, its metric and score calculations are based mainly on how much energy a building uses rather than on the environmental impact of the particular type of energy used. This means that if Building A, powered by fossil fuels, uses less energy than Building B, an identical building powered by low-impact hydroelectric energy, Building A could earn a higher score, even if its carbon footprint might be greater.

For more information on green power and how it affects your metrics, please see the ENERGY STAR Technical Reference on Green Power [PDF – 333 KB].

Learn more

For more information, please see the following resources:

• ENERGY STAR Technical Reference on Source Energy [PDF – 369 KB], U.S. Environmental Protection Agency
• The difference between source and site energy, U.S. Environmental Protection Agency