Personal Energy Meter Methodology
The Personal Energy Meter aims to help United States residents gauge the carbon dioxide emissions that result from their personal household and travel decisions. Users can see how they measure up against others across the nation or in their region of the country.
The National Average
In this calculator, the average U.S. citizen is estimated to be responsible for about 10 metric tons of carbon dioxide emissions (CO2) per year. (Each metric ton is 2,204.6 pounds.)That counts only direct emissions, those that result from personal choices.
Total U.S. emissions per capita—including industrial and commercial emissions—add up to much more: 19.2 metric tons of CO2, based on the 2008 Emissions of Greenhouse Gases Report from the U.S. Energy Information Administration (EIA) (a total of 5.8 billion metric tons of CO2) and the U.S. Census Bureau’s population estimate of 304.4 million people.
But the EIA report specifies that only 21 percent of the emissions are residential. That amounts to 4 metric tons per person in home energy emissions.
To determine average carbon emissions for personal car travel, which are not categorized by EIA, we used Federal Highway Administration statistics. The total of miles traveled in 2008 by passenger vehicles was 2.7 trillion. Divided by 208 million licensed drivers in the United States, that amounts to 13,078 miles driven on average. Because the average U.S. fuel economy is 22.6 miles per gallon, that translates to 579 gallons on average per driver. The EIA reports that the carbon factor for gasoline is 19.6 pounds of CO2 per gallon, so that adds up to 11,321 pounds of CO2 per driver, or about 5 metric tons.
For average emissions from air travel, we used the U.S. Bureau of Transportation Statistics’ 2008 figure of 583.5 billion miles traveled by air carrier. There are no reliable data on how many U.S. residents fly, but if the number of licensed drivers is used as a proxy for the population that uses air travel, that translates to 2,801 miles per person.
The Greenhouse Gas Protocol developed by World Resources Institute and the World Business Council for Sustainable Development estimates that carbon emissions per mile for air travel range from 0.11 to 0.18 kilogram per kilometer, or 0.40 to 0.65 pound per mile, depending on the length of the flight. (Shorter flights result in greater emissions per mile because a larger portion of the trip is spent in the energy-intensive takeoff and landing.) The estimate of 1 metric ton CO2 per person average U.S. emissions assumes flights are divided between long and short.
Regional averages are derived from EIA’s most recent data for greenhouse gas emissions by state, which are for 2007. These were divided by the census population estimates by state for 2007 to arrive at a per capita figure. As above, it was assumed that 21 percent of those emissions were from home energy use, in line with EIA’s breakdown on residential emissions. The data are not available to account for states where a much larger percentage of carbon emissions are from industry. Regional averages may be high in states like Alaska and Wyoming, with small populations and large energy industries.
There are no reliable per capita data on regional differences in personal auto or air travel.
The Personal Score
Carbon emissions from electricity vary significantly depending on your region of the country, because some areas rely more heavily on carbon-intensive coal for power. We obtained the most up-to-date state electricity carbon factors—how many pounds of carbon dioxide per kilowatt-hour—from the EIA based on the most recent year of data, 2008. We used regional factors that took into account the carbon intensity of the power generated not just in each state but in the entire portion of the grid—sometimes crossing several states—from which your region draws its power. A list of these carbon factors is here.
When you enter how much you pay for electricity, EIA’s most recent electricity prices in your state are used to determine how many kilowatt-hours of electricity you use per year. Use is multiplied by your region’s carbon factor in order to get your electricity emissions.
For other fuels, the process is similar, using EIA’s most recent prices for natural gas, heating oil, and propane. But the carbon factors for these fuels do not vary by region of the country; they are the national averages recorded by EIA: 120.593 pounds of CO2 per thousand cubic feet of natural gas, 26.033 pounds of CO2 per gallon of heating oil, and 12.805 pounds of CO2 per gallon of propane.
If you use renewable energy, your score is reduced by the percentage of carbon-free power that you entered.
For auto emissions, the meter simply divides miles traveled per year by vehicle fuel economy to determine the gallons of gasoline you use per year. That number is multiplied by the EIA’s current carbon factor for motor gasoline of 19.6 pounds of CO2 per gallon.
For emissions from flying, short flights are assumed to be 300 miles and long flights, 2,500 miles. The meter multiplies the number of flights by miles. The number of miles traveled is then multiplied by the emissions factors of 0.4 pound of CO2 per mile for long flights and 0.65 for short flights—taken from the Greenhouse Gas Protocol as explained above.
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