After some fun times manipulating spreadsheet data from the Energy Information Administration (EIA), the data arm of the Department of Energy, and the Environmental Protection Agency (EPA), we were able to determine grid carbon intensity and average residential price numbers as follows:
These are based on average numbers from 2003-2007 (more recent data is not yet available). If you would like to download the original data sets, you can download EIA data here and EPA emissions data here. You may notice Idaho and Vermont have almost no grid emissions: this is because Idaho is largely hydro powered, and Vermont is almost entirely Nuclear and hydro powered. California has the third cleanest grid, powered largely by natural gas, nuclear, and hydro.
The numbers above enable us to determine the component of your carbon footprint due to electric use. While there will be some variation in your electric rates, our tests show the numbers above get us within 10 percent of direct energy use measurement, and asking you for your bill is a lot easier than asking you to average all your monthly kWh usage.
If you use electric heat, the emissions from it will be captured in your electric use above. If you do not use electric heat, we can calculate your emissions based on your fuel source. Fuel Oil emits 161.4 pounds of carbon dioxide for every mmBtu worth of fuel you burn , and natural gas, propane, butane, and other gas-based systems of heat produce only 117.1 pounds of carbon dioxide for every mmBtu worth of fuel you burn . The efficiency of your furnace and ducts will be captured by how much fuel you have to buy, and your pumps and fans will already be included in the electric bill footprint. Thus, using your type of heating source and how much you spend on fuel, we can calculate the carbon contribution from your heating use.
You may have noticed that not much happens when you change the radio button between oil and natural gas, even though natural gas releases less carbon dioxide than oil. This is because even though natural gas is cleaner, it is also less expensive; $100 gets you a lot more natural gas than it does oil, so unless you change both the heating source and how much money you spend on heat, you will not see much of difference in your carbon footprint. Using more of something clean is as bad as using less of something dirty in this case.
Lastly, your carbon footprint due to car travel is also relatively easy to calculate. We know burning one gallon of gasoline emits 19.56 pounds of carbon dioxide , so once you tell us how far you drive and the efficiency of your car, we can immediately figure out your carbon emissions from driving.
To determine the carbon impact from your air travel, we used IPCC data to calculate the average emissions from jet aircraft per mile per passenger. Our conclusion: 0.462 pounds of carbon dioxide per person per mile. Thank you IPCC . You tell us the miles, we tell you the carbon.
Add it all together, and you have your total carbon footprint. Offset your carbon footprint today!
[1, 2, 3] MIT has a thorough compilation of useful energy contents and unit conversions available here.
 You can download the IPCC report here.