How We Calculate Your Carbon Footprint
carbonfund.org/how-we-calculate
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Carbonfund.org's Carbon Calculators use information from the U.S. Department of Energy's Energy Information Administration and other leading sources to develop an accurate assessment of carbon dioxide emissions emitted per energy type or use.