Basic Building Characteristics
The amount of energy a building uses is determined by a number of factors. These include:
- Type and function of building
- Size of building
- Efficiency of building equipment and systems
- The number of people in the building and its hours of operation
- Operation and maintenance practices
Age and construction characteristics
Annual Electricity and Natural Gas Use
Electricity and natural gas use comes from utility bills.
Site and Source Energy Intensity
Site energy intensity is the amount of energy (electricity and natural gas) consumed per square foot of gross building area. This energy is measured by meters at the site. To show electricity and natural gas together, they are converted to a common unit: British thermal units (BTU). There are 3,412 BTU per kilowatt-hour (kWh) of electricity. There are 100,000 BTU per therm of natural gas.
Source energy intensity takes into account the fuels consumed in the generation, transmission and distribution of electricity, as well as the energy losses from storing, distributing and dispensing natural gas. The national average is that for every BTU of useful electricity used at the point of end use, 3.34 BTU of energy were consumed to create and deliver that electricity. For natural gas, the ratio is 1.047, meaning that slightly more than 1 BTU of energy was actually needed for each 1 BTU used by the end customer. Two buildings may have the same site energy intensity but different source energy intensities if they use a different ratio of natural gas and electricity.
For example, a typical small commercial building might be 5,000 sq. ft. and consume 50,000 kWh of electricity and 2,500 therms of natural gas in a year.
The site energy intensity is calculated as:
For the same building, the source energy intensity is calculated as:
We calculate the carbon footprint using well-established emission factors for electricity and natural gas. According to U.S. EPA eGRID information for Dominion Virginia Power in 2007, the electricity generation process results in 1.19 pounds of CO2 emissions per kWh consumed. This 1.2 pounds of CO2 consists of a generation mix of nuclear power, coal, natural gas and small amounts of fuel oil and hydro power. According to the U.S. Energy Information Administration, each average therm of natural gas consumed results in 11.7 pounds of CO2 emissions. The carbon footprint for each building, then, is calculated based on the amount of electricity and natural gas consumed, multiplied by the emissions factor for each fuel.
Although eGRID has released a more recent emissions factor, the Report Cards use the same factor throughout to show the effect of changes in energy use, rather than changes in emissions factor. Our latest greenhouse gas emissions inventory uses the appropriate emissions factors for each year. See the inventory in Quick Links for how the change in emissions factor impacts our inventory.
EPA ENERGY STAR® Ratings
ENERGY STAR Ratings are currently available for a limited number of building types. These ratings compare energy use and characteristics at your building to buildings across the country. Accounting for differences in weather, size, occupancy and operating hours, buildings are assigned a score from 0-100. Buildings that earn a 75 or higher are eligible to receive an ENERGY STAR certification.
Buildings use EPA’s Portfolio Manager software to benchmark their energy use and see if they qualify for the ENERGY STAR. Arlington tracks all of its buildings in Portfolio Manager, but only a few of them (offices and warehouses) are eligible to receive an ENERGY STAR certification. ENERGY STAR continues to add more eligible building types as data is available.
For more information, please see ENERGY STAR.