Bear in mind that this goes on 24 hours a day, seven days a week, and 365 days a year. Are you beginning to acquire a sense of the amount of power required to keep the web hosting company up and running with no downtime? Ah yes, downtime. I almost forget about that. In an attempt to reduce failure, a lot of web hosts use what is called "redundancy" in which they have a matching server, sometimes in a different place, in case the first server fails. Therefore it is not purely one web server using up all that electricity; there are more often than not two. And if they are not making use of redundant servers, they have some other hardware in place to defend the data like tape or hard drives.
These web hosting companies claim to become the greenest of the group. The acquire 100% of their authority from solar panels and wind farms. I have not heard of one making use of hydro as of yet. In order to obtain the most out of their electrical energy, they also use hardware that does not need as much power to work. By making use of energy efficient hardware, the companies need less total electricity to stay in operation. A green web host that is not dependent on a major power company is more than potential to have a green mindset during the whole organization.