Google picks offsetting projects that are more valuable than average; details are here: http://static.googleusercontent.com/external_content/untrust...

I saw a talk recently about how much solar and wind power Google buys, and it's not at all cost effective. Google does it because they think it's cool, not because it saves them money. (But, the rate of return on the investment into wind infrastructure tends to be pretty good; wind power can be sold at a higher price than regular power.)

As an example, look at the Googleplex in Maps some time; all the parking spaces are covered with solar cells to charge the specially-converted fleet of plug-in-hybrids available for employees to borrow during the day. Building solar cells and custom-modding cars to give to employees for free is not exactly "saving them a lot of money". But it sure is cool.