Carbon dioxide is one of the major contributors to global climate change and many communities are seeking to reduce their carbon footprints over time. To determine the carbon impact of buildings in a scenario, we can measure the amount of carbon dioxide emissions per household or employee per year from building energy use. The amount of carbon dioxide produced depends on the energy sources used for heating and electric systems. Coal emits more carbon dioxide per unit of energy than natural gas, while wind and solar emit no carbon dioxide at all.

The Energy Information Administration provides values for tons of carbon dioxide emitted per million BTUs of energy use for each energy source (coal, propane, natural gas, etc.). The local mix of energy sources is used to determine the local value for carbon dioxide emissions, which is then applied to the building energy use figures. The user is able to easily change assumptions to better calibrate results for their locality.