Our calculations use the median webpage weight from the HTTP archive as main reference. We calculate the difference between a website page weight and that median webpage weight and convert it into a percentage. Our CO2 calculations use the methodology from Sustainable Web Design

"We used these data points to define the calculations below:
Annual Internet Energy: 1988 TWh
Annual End User Traffic: 2444 EB
Annual Internet Traffic / Annual End User Traffic = 0.81 TWh/EB or 0.81 kWH/GB
Carbon factor (global grid): 475g/kWh

These are the formulas we came up with:
Energy per visit in kWh (E): E = (Data Transfer per Visit in GB x 0.81kWh/GB x 0.75) + (Data Transfer over the Wire (GB) x 0.81kWh/GB x 0.25 x 0.02)
Emissions per visit in grams CO2e (C): C = E x 475g/kWh"

As the Internet is global, we use the global grid as a carbon factor. But in the future we plan to add the use of data centers powered by renewable energy into our equation.