How does the calculator work?

Frequently Asked Questions

 

Electricity

The calculation to determine the emissions from your electricity use is different depending on whether you: (a) choose to enter your actual electricity consumption or (b) choose to use the national average consumption.

Entering your own information
When you enter your own annual kWh consumption, you are asked to enter the zip code of your home or office because electricity production in different regions of the country produces different levels of pollution according to the energy mix present in the region’s grid.  For instance, the average emissions rate in the Western US is 1.11 lbs/kWh, while in the Upper Midwest the rate is 1.82 lbs/kWh.   See FAQ titled “Why does the number of Green Tags I need depend on where I live?"

The calculator multiplies your annual kWh (let's say 10,000) by the average carbon dioxide factors for your region1. For instance, if you reside in the state of Minnesota, a state in which electricity is produced primarily by burning coal, your carbon dioxide factor is

One more point: Transmitting electricity is inefficient and imperfect.  There can be a line loss of as much as 9% when electricity is transmitted through the system2. Therefore, we add a multiplication factor of 1.09 into the equation. For 10,000 kWh of actual use, 10,000 x 1.09 = 10,900 kWh must be generated. To complete our Minnesota example:

Although we use regional averages to make these calculations, we recognize that an average figure does not exactly replicate the emissions caused by the electricity used in your household. For a more accurate representation of your actual emissions, contact the utility that supplies your power.

Choosing the national average
The average U.S. home uses 11,040 kWh/year3. Since 9% of the energy generated is lost in the transmission system, the average use actually causes 11,040 x 1.09 = 12,034 kWh to be generated. We multiply that number by your region’s emissions factor, or if you choose not to supply your zip code, we use the national average emissions factor of

.

For example if you’ve provided your zip code and happen to live in Minnesota, your factor is , so your emissions would be…


These emissions calculations follow the guidance of The Green-e Climate Protocol for Renewable Energy v1.0

Home Heating

This section asks you to pick the type of fuel you use to heat your home or office. If you heat with electricity, then your heating energy use is already included in the electricity calculation you did previously. If you heat with another fuel, you are asked to select natural gas, oil, or propane.

Once you've selected the fuel, you can choose to enter your own information based on what you actually consumed over a year, or, you can choose to use the national average.

Entering your own information
Based on the fuel source you choose, the calculator computes the emissions by multiplying the appropriate emissions factor and the number of units you used (gallons, CCF, or Therms) for each fuel.
Emissions factors for each fuel type are4 : 

For demonstration, if you use 800 CCF of natural gas annually, then:


Choosing the national average
The average U.S. home uses one of the following:

NOTE:  Each average includes only the set of households using that particular fuel

 

Driving

The calculation for auto travel works differently depending on whether you choose to enter you own annual miles driven, and your own miles per gallon, or you choose to use national averages.

Entering your own information
The first step is to enter the miles per gallon for your vehicle (let's say 25).
Then enter your own annual miles driven (let's say 10,000).
The calculator then divides your annual miles driven by your miles per gallon- in this example:

The calculator then multiplies your fuel consumption by 19.564, which is the amount of CO2 released by burning one gallon of gasoline­4.

Choosing the national average
The calculation is performed the same as above, but uses the following information:
The average annual distance driven per vehicle in the U.S. is 12,578 miles8 .
The average car gets about 25.9 miles per gallon, the average SUV gets 19.7 miles per gallon9 .

 

Flying-General (Emissions from a single passenger)

The average passenger miles per gallon for a domestic or international trip originating in the U.S. equals 33.4, according to “Transportation, Energy, and the Environment”, Section A - U.S. Energy Consumption and Transportation Sector Energy Consumption, Table 4-21. (Note that this is not much better than the average automobile, if driven with only one occupant.)
Burning a gallon of jet fuel releases 21.095 lbs4 of CO2.  Combining these two factors:

Air travel also releases significant amounts of non-CO2 greenhouse gas emissions, and their impact is magnified by the fact that they are released directly into the upper atmosphere.  The effect of these gases is accounted for by a factor called the radiative forcing index (RFI). 

In its 1999 Special Report on Aviation in the Global Atmosphere, the Intergovernmental Panel on Climate Change (IPCC) estimated the RFI from air travel in 1990 to be between 2 and 4, averaging 2.7 times the carbon impact alone. More recently, the TRADEOFF project of The Fifth Framework Programme of the European Commission of the EU, suggested an RFI of 1.9. The Climate Neutral Network (CNN) conducted significant research on this topic, and after considerable discussion, BEF and CNN agreed that the appropriate RFI factor is 2.  Hence, our calculator doubles the 0.63 lbs of CO2, with a result of 1.26 lbs of total CO2 equivalents (or CO2e) per passenger mile.

Based on the Climate Neutral Network’s analysis, an additional 8% has added to the total to account for the emissions associated with the upstream refining of jet fuel. The result is that 1.36 lbs. of CO2e are created for each passenger mile traveled (0.63 x 2 plus 8%).   It is important to note that many carbon calculators on the Internet, do not account for these additional emissions and hence, significantly underestimate total Greenhouse Gas emissions.

What is CO2e? 

CO2e is shorthand for carbon dioxide equivalent.  Many people know that carbon dioxide (CO2) is a leading cause of global climate change. However, CO2 isn't the only guilty gas. Methane for instance has more than 20 times the climate change impact of CO2. When we talk about the climate impacts of a given activity, it is important to have a single term and a single unit of measure. For global climate change, "Carbon Dioxide Equivalents," or CO2e is a widely used standard.

The concept of CO2e is frequently applied to aircraft emissions.  Emissions from air travel need special consideration when we try to quantify their effect.  Because aircraft exhaust is emitted directly into the upper atmosphere, the contents of the exhaust interact differently with atmospheric gases than they would if they were emitted at the earth’s surface- the severity of these interactions is quantified by a factor referred to as radiative forcing. 

As an example, the exhaust from an airplane trip may contain 4.3 lbs of NOx, 4.8 lbs of CO, 1.0 lbs of other hydrocarbons, and 1500 lbs of CO2.  Because the non-CO2 contents are more potent and react differently at high altitudes, the total impact of that trip has the equivalent climate change impact of releasing 3000 lbs of CO2 at the earth’s surface.  In order to include the equally important impact of the non-CO2 emissions in our calculations we refer to the total in units of CO2e.  This unit helps us to quantify how much CO2 we need to offset with Green Tags in order to mitigate the effects of all the emissions from the trip.  Many other web calculators ignore the impact of non-CO2 emissions and hence under-estimate your actual climate change impact.

What is RFI? 

Before we start explaining, let’s define some terms:

Climate change refers to temperature changes experienced at the earth’s surface as a result of chemical changes in the composition of the earth’s atmosphere.  These atmospheric changes affect the amount of solar energy that travels to and from the earth through the atmosphere.  When the amount of energy reaching the earth exceeds the amount leaving the earth, the net effect is warming.  

Radiative forcing refers to how much a particular activity contributes directly to the atmospheric chemical reactions that cause climate change. A higher radiative forcing index (RFI) means an activity has a greater contribution to chemical changes in the atmosphere.  All human-influenced, or anthropogenic, emissions (i.e. automobiles, electric power plants, air travel, etc.) have an RFI associated with them.

Now, on with the explanation. 

We know that burning fossil fuels results in emissions of carbon dioxide.  Burning of fossil fuels also results in emissions of SOx, NOx, CO, and particulates called soots or aerosols.  When released at the surface of the earth, these substances are widely dissipated and thus have a lower net impact on chemical reactions within the atmosphere when compared to carbon dioxide, which rises into the upper atmosphere.

Aircraft get special attention because they commonly operate in parts of the atmosphere known as the Upper Troposphere (UT) and the Lower Stratosphere (LS).  This is significant because many of the chemical reactions that dictate the insulating properties of our atmosphere occur in these two regions.  When SOx, NOx, CO, water vapor, and aerosols are released directly into the UT and LS as a result of aircraft fuel combustion, they have immediate effects on the regional atmospheric chemistry.  It is important to note that the effects of these compounds in the upper atmosphere are greater than they would have been if emitted at the surface.

NOx emissions in the UT and LS result in an increase in ozone production.  Ozone, like CO2 and methane (CH4) is a natural component of our upper atmosphere, and while it helps shield us from harmful UV rays, it is also a greenhouse gas.  In other words, increases in ozone in the upper atmosphere lead to net warming of the earth.  

SOx emissions are also a result of jet fuel combustion.  Chemically speaking, they actually lead to a decrease in UT and LS ozone, BUT they also lead to production of sulfates and aerosols.  Both of these substances serve as potential sites for atmospheric reactions that negate the impact of the ozone decrease and result in net warming.  

You’ve probably seen a jet flying overhead and noticed a white line trailing it across the sky.  That line is called a condensation trail, or contrail for short.  Contrails are comprised of water vapor and various particles (aerosols).  Persistent contrails can linger in the sky for extended periods of time and actually have a direct warming effect.  Even if they dissipate quickly, the water vapor and particles can contribute to the formation of cirrus clouds.  

While thick cumulus clouds block solar energy from reaching the earth’s surface, high, thin, cirrus clouds act like a one-way door, allowing solar energy to reach the surface, and then reflecting it, or trapping it there.  Clearly, cirrus clouds can, and do, occur naturally, and should hardly be considered a problem as part of natural atmospheric cycles.  However, emissions from aircraft can ultimately lead to an increase in cirrus cloud coverage.  When compared to the cloud coverage that would exist in the absence of aircraft emissions, the net effect is warming of the earth.  

Rolling all of these factors together, we use the RFI to quantify how large the total impact of aviation emissions is on climate change.  Generally, the RFI of non-aviation emissions is approximately one.  The IPCC (Intergovernmental Panel on Climate Change) estimates that the overall radiative forcing by aircraft emissions (excluding that from changes in cirrus clouds) is a factor of 2 to 4 times larger than the radiative forcing by aircraft carbon dioxide alone. What that means is that the total climate change impact of aviation can be more accurately quantified by multiplying the carbon dioxide emissions value by a factor of 2 to 4.  BEF and the Climate Neutral Network have mutually agreed to use a conservative factor of 2.  Many carbon calculators on the Internet ignore RFI and hence, understate users’ contribution to climate change.

 

Green Your Event

The data used in our Event Mini Calculator to determine the number of Green Tags needed to green your event is from the U.S. Department of Energy, Energy Information Administration- Commercial Buildings Energy Consumption Survey (2003).

For example, if you wanted to offset the CO2 from an event at a Convention Center:

If you would also like to offset your ground or air travel (or other attendees' travel) to and from the event, you can go to our easy-to-use Travel Mini Calculator.

 

Frequently Asked Questions


How do you calculate the # of Green Tags I need to buy?

The number of Green Tags you need is determined by taking the total CO2e that you produced (46,284 lbs CO2e in the example above), and dividing it by the amount of CO2 offset by each Green Tag.
If you choose to buy Denali Green Tags, 46,284 lbs. of greenhouse gases produced (in this example), divided by 1,500 lbs. of greenhouse gases avoided = 33.06 Green Tags.

If you choose to buy 100% Solar Green Tags instead, your offset value would be 46,284 lbs of greenhouse gases produced divided by 1295 lbs of greenhouse gases avoided = 35.7 Green Tags.


How many Green Tags do I need to buy to offset my climate impact?

The number of Green Tags you need is determined by taking the total CO2e that you produced (46,284 lbs CO2e in the example above), and dividing it by the amount of CO2 offset by each Green Tag.

If you choose to buy Denali Green Tags, 46,284 lbs. of greenhouse gases produced (in this example), divided by 1,500 lbs. of greenhouse gases avoided = 33.06 Green Tags.

If you choose to buy 100% Solar Green Tags instead, your offset value would be 46,284 lbs of greenhouse gases produced divided by 1295 lbs of greenhouse gases avoided = 35.7 Green Tags.


How are your average emissions for each category calculated?

Electricity
The average carbon dioxide (CO2) emissions from one kWh of electricity generated by utilities in the U.S. is 1.36 lbs1.
The average U.S. home uses 11,040 kWh of electricity per year3.  About 9% of the energy generated by the utilities is lost in the transmission system before it reaches the home2, so the average use of 11,040 kWh actually requires about 12,034 kWh to be generated.

Home Heating
To simplify this calculation we assume the home is heated with natural gas.
Burning one CCF of natural gas releases 12.0593 lbs CO2.4
The average U.S. home uses 756 CCF/year.5


Auto Travel
The average miles driven per vehicle per year in the U.S. is 12,578.8
The average automobile gets about 23.7 miles per gallon9. (Most SUVs get less.)

The calculator then multiplies your fuel consumption by 19.564, which is the amount of CO2 released by burning one gallon of gasoline4.

The Total
Electricity = 16,366 lbs. per year of CO2
Home Heating = 9,792 lbs. of CO2 per year.
Auto Travel = 10,383 lbs. of CO2 per year.
Total = 36,541 lbs. of CO2 per year.

How accurate is the calculator?

We use the most trusted data available to determine the environmental impact of burning fossil fuels, relying on analysis done and values provided by the most commonly accepted and reliable sources, such as the EPA, the Department of Energy, the Bureau of Transportation Statistics, and other U.S. government agencies. Still, when the results of these analyses are applied to any single individual or household, the resulting number will be an approximation.

The area with the widest variation will be your electricity use, because that is dependent on what types of fuels are used by your particular utility. We use each region’s average data. If you want more accurate data, contact your utility and ask for their energy mix and emissions output information.

Why did you ask me for my zip code?

Different regions of the country have different mixes of electricity. (see GREEN TAGS FAQs)  Our calculator uses the average emissions for each region as published by the North American Electric Reliability Council (NERC) to determine the air pollution generated by your electricity consumption. If you want information for your specific utility please contact the utility or your region’s utility regulatory agency.

 

Copyright © 2008 Bonneville Environmental Foundation

1 eGRID2006 Version 2.1 (April 2007) Year 2004 Summary Tables- Year 2004 NERC Region Emissions

2 Energy Information Administration- Residential Sector Energy Consumption, Section Note: Electrical System Energy Losses

3 Energy Information Administration- Table 5: U.S. Average Monthly Bill by Sector, Census Division, and State 2006

4 Energy Information Administration- Instructions for Form EIA 1605B, Voluntary Reporting of Greenhouse Gas Emissions, Appendix B

5 Energy Information Administration- Natural Gas Residential Choice Programs- U.S. Summary 2006

6 Energy Information Administration- Residential Energy Consumption Survey- Table 2. Fuel Oil Consumption and Expeditures in U.S. Households by End Uses and Census Region, 2001

7 Energy Information Administration- Residential Energy Consumption Survey-Table 4. LPG Consumption and Expeditures in U.S. Households by End Uses and Census Region, 2001

8 U.S. Department of Transportatioon, Federal Highway Administration, Highway Statistics 2005, Tables DL-1C and VM-1, and annual reports dating back to 1996.

9 U.S. Environmental Protection Agency, Light-Duty Automotive Technology and Fuel Economy Trends:  1975 Through 2006, July 2006.

10 See FAQ titled: How do you calculate the emissions from a single airline passenger?

11 Bureau of Transportation Statistics- T-100 Domestic and International Market and Segment Database

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