How Was It Calculated

General Information

Location: Electricity is generated from different types of power plants, such as coal, natural gas, and hydroelectric. Different types of power plants create different amounts of carbon dioxide per kilowatt hour of electricity produced; therefore we need to know the user's location to apply the correct calculation.

Size of household: Because many households have more then one person, and energy use is generally recorded for the household as a whole, the calculator allows the user to input number of people in the household. Children will generally have a smaller impact on carbon emissions than adults, thus the calculator suggests including only children of driving age or those who have active schedules. Final energy emissions are divided by the number of people in the household to get the per capita emissions.

Detailed inputs: For many of the calculations, we provide default data for people who don't know their exact energy use. We recognize, however, that some people do know this information and prefer more detailed and accurate calculations, and we allow for detailed inputs to be used. The defaults and detailed inputs are both explained in the sections that follow.

Transportation

Automobile: Inputs are for number of miles driven and fuel efficiency of the cars, or alternatively the make, model, and year of the cars. If make, model, and year are given we derive the fuel efficiency of the car from a database provided by TerraPass.

Total number of miles driven is divided by fuel efficiency (miles per gallon) of the vehicles to determine total number of gallons of fuel that are used. Number of gallons of gasoline is multiplied by the emissions factor of 19.6 to convert to pounds CO2, or 22.4 for diesel vehicles. These emissions factors come from the Energy Information Administration, Fuel and Energy Source Codes and Emissions Coefficients.

Airplane: Air travel per-mile emissions are significantly affected by the length of the flight because a high percentage of fuel use and emissions are expended on take-off. Therefore we ask for number of short, medium, long, and extended flights. The default input is simply the number of each type of flight, defined as each leg of a flight such that a round trip flight with one stop each way has four legs. If this default is used we estimate an average length of 250 miles for a short flight, 800 miles for a medium flight, 2500 miles for a long flight, and 5000 miles for an extended flight.

Different emissions factors are used for each flight length, as follows:

Short flight: .64 lbs/mile
Medium flight: .45 lbs/mile
Long flight: .39 lbs/mile
Extended flight: .39 lbs/mile

The total number of miles for each type of flight is multiplied by the emissions factor for that type of flight to get pounds of carbon dioxide, which are then added together to get the total carbon footprint from air travel.

Air travel definitions and factors are from the GHG Protocol Mobile Combustion Tool. The emissions factors for short and long haul flights are originally from the UK Department for Environment, Food and Rural Affairs (DEFRA). The emissions factor for medium haul flights was derived, using an assumed distance of 1600 km and the following formulae based on Emission Factors provided by DEFRA.
X+452y =3D.18*452 for short haul
X+6342y=3D.11*6342 for long haul

Home Energy

Electricity: For users who provide an estimate of their electricity bills, we take the mid-point of the estimated range and divide by the cost of electricity in their state (provided earlier in the calculator), which we get from the Energy Information Administration (EIA), to come up with the electricity used. We multiply this number, or actual electricity use if provided by the user, by the emissions factor for the user's state, provided by the EIA. This factor is different for each state because power sources are different; some states are more dependent on carbon-heavy sources such as coal, while others use a higher percentage of sources that don't produce carbon such as hydroelectric or nuclear power.

Green power subscriber: If green power sources that do not result in carbon emissions are used, the calculator reduces the carbon output due to electricity by the percentage of green power used. For example, if 25% of the user's electricity use is supplied by green power, the carbon emissions due to electricity are reduced by 25%.

Other fuels (natural gas, heating oil, and propane): As with the electricity calculation, users can provide an estimate of how much they spend on other fuels, or can provide the exact fuel usage. For users providing a cost estimate, we use the mid-point of the estimate range and divide by the cost for the user's state, provided by EIA on different pages for natural gas, heating oil, and propane.

For each type of fuel we use the following emissions factors, provided by EIA:

Natural gas: 11.7 lbs carbon dioxide per 1000 ft3
Heating oil: 22.4 lbs carbon dioxide per gallon
Propane: 12.7 lbs carbon dioxide per gallon.

Results

The carbon dioxide footprint is the sum of all the above calculations, resulting in an estimate of the carbon dioxide that is produced by home energy use and transportation. The result is compared against an estimated monthly per capita U.S. average of 1,250 pounds resulting from transportation and home energy emissions. This amount represents roughly 32% of per capita emissions in the U.S.

For comparison purposes, any result within 33% of the 1,250 pounds per month is considered to be an "average" amount. Between 33% and 67% above or below the 1,250 pounds per month is considered "more than average" or "less than average," respectively. A result greater than 66% above or below the 1,250 pounds per month is considered "much more than average" or "much less than average," respectively.

A simpler way to consider comparative results is in the following table: