In reply to Andreas Nurbo.

The energy source used is just one of the many factors. It is not the only one, nor is it the most important one.
A website that has 10MB of images on its frontpage will produce a lot more carbon than an optimized 30kb mostly-text site – even if the 1st one is powered by solar power and the 2nd one by burning coal.
The metric is an indication, an estimation. Nobody can accurately measure the carbon footprint of a website. The numbers are meant as a guide, not a gospel.
If a website produces 2g/visit then it’s bad and they should do something about it ASAP. There’s lots of room of improvement there! If they do 0.1g then it’s pretty good, but I’ve managed to get as low as 0.01 on a site. There’s always something that we can do to improve things.