It really seems like its another company jumping on the green bandwagon. Unless you sell "green tools" or sell to communes that try to be 100% green, its just a tacky addon to a website.
I mean, I really do not think anyone cares. I dont think the datacenters are especially "green" especially if they ever have to use catapiller generators, heh.
Also think of the backup website you are going to have to buy to display this when your site is down. "Sorry, the sun has not been out much for 3 days, so our solar power generator is not working. The site will be back up when the sun shines. PRAY FOR SUN PLEASE!"
I've been following CO2Stats for a while. They really do use rigorous analytics and vetted renewable energy. I once saw a talk by one of the founders that was pretty convincing, and they couldn't get their impressive client list without something more than marketing.