We recently added the 2011 model year cars to our carbon footprint calculator, and it now includes all-electric vehicles. As this is a first, this post is dedicated to the calculation details because they aren’t as straightforward as a gasoline or diesel car.
First, for comparison: a traditional car’s footprint is calculated by multiplying the quantity of fuel used in a year by an “emissions factor,” the pounds of CO2 emitted by combustion of one gallon of that fuel. To arrive at annual fuel consumption, the TerraPass calculator multiplies the US EPA’s combined City – Highway miles per gallon figure for your car by the number of miles you tell us you’ll drive.
We keep this calculation simple. We don’t attempt to count the full life-cycle emissions of the fuel, which would include factors for losses during extraction, refining, and transportation from the oil reserves to your car. We don’t account for incomplete combustion in your engine or fugitive emissions from your tank into the air. We don’t account for your driving habits or routes, which materially affect your miles per gallon. We keep it simple.
Enter cars which are fueled at least in part, by grid electricity.
Vehicles which plug-in introduce at least two complications to the footprint calculation. First and most obviously, is the CO2 emissions factor. For electricity, this factor depends on the fuel used to run the power plant. Regions with a heavy dose of coal-fired power have a much higher CO2 emissions factor than regions which rely on natural gas or hydro power. We have chosen to use the US national average grid emissions factor, as reported by the US EPA’s eGRID database. You can see how your region compares with the national average on this one-pager.
Second, how much electricity does the car require? For motor vehicle labelling purposes, the EPA decided to compare alternative fuels to gasoline so that car buyers can compare efficiency and costs across various fuel types. Using standard energy conversion factors, the heat energy available in a gallon of gasoline equals the heat energy available in about 33.7 kilowatt-hours of electricity. To get a familiar “miles per gallon” figure, the EPA drives plug-in cars through its standardized cycle and measures the energy used. They convert the measured kwh to gallons using the 33.7 multiplier and arrive at a “miles per gallon-equivalent” figure.
So the TerraPass calculator multiplies the “mpg-e” figure by the national average electricity CO2 emissions factor, and then multiplies again by the number of miles you tell us you’ll drive. As with liquid-fuelled cars, we don’t take into account the full life-cycle emissions of the electricity; we don’t count electricity transmission line losses, the life-cycle of the power plant fuel, or other upstream factors.
The only catch to our new electric vehicle calculations is that you’ll need to enter your mpg-e directly into the calculator. This is due to the structure of the underlying US EPA vehicle database, which doesn’t yet include electric vehicles.
What about plug-in hybrids, you ask? Blog post on that, more complicated, topic will follow.