Let's connect the dots by looking at an example.
Suppose a utility bill shows a usage of 10,000 standard cubic feet (SCF) of natural gas.
As we said earlier, the combustion of natural gas emits three greenhouse gasses:
For this example, the following three emissions factors are required for the three emitted GHGs:
To calculate emissions, the amount of natural gas must be multiplied by each emissions factor.
Our activity data is in standard cubic feet and the emission factors are in mmBtu.
So the first step is to convert the activity data into the same units as the emission factors.
We do this by multiplying the activity data by a conversion factor.
Now the activity data can be multiplied by each emission factor. The mmBtu units cancel out, leaving only greenhouse gas emissions.
We now have quantities of each greenhouse gas emitted due to the footprint activity (burning natural gas). Unfortunately, we can’t simply add the three numbers together to come up with a total emissions. Each of these greenhouse gasses is not equal in terms of its impact on global warming.
In order to correctly total these three emissions quantities, they must first be converted into a common unit of carbon dioxide equivalent (or CO2e). This is accomplished by multiplying each quantity by its global warming potential (GWP).
Now that each of the emissions is in a common unit of CO2e, they can be totaled to provide a single value representing the greenhouse gas that was emitted as a result of the original footprint activity (combusting 10,000 SCF of natural gas).
So, to recap...
This is our complete calculation:
This calculation is intended as an example and numbers may be off due to rounding.