What is an emission factor?

The arithmetic is trivial: 10,000 kWh × 0.412 = 4,120 kg CO2e. What is not trivial is that the same activity has a different factor in a different place, a different year, and sometimes a different state within the same country.

A factor is only meaningful with four things attached: what activity it converts, where it applies, which year it describes, and who published it. A factor without a vintage is a factor you cannot defend, because grids change every year.

Factors also come in tiers of trust. A supplier’s own measured figure beats a national published average, which beats a regional average, which beats a global default, which beats a spend-based proxy that converts dollars to emissions. Reporting frameworks expect you to say which tier you used.