Good question.  Buying stock in PBW is an entirely different thing than buying a TerraPass.  You're welcome to do either or both, depending on your environmental goals.

PBW is an mutual fund that allows you to invest in companies that produce or promote clean energy.  When you make a socially conscious investment, your money doesn't actually go to the companies owned by the fund.  Although the companies may see a secondary benefit from the purchase of their stock, mainly the benefit is to the person making the investment, who is seeking a return on his or her money.

When you buy carbon offsets through TerraPass, you are actually directly purchasing a guaranteed reduction in carbon dioxide emissions.  The bulk of the purchase goes directly to the companies who produce clean energy through wind, biomass, or other means.  These companies use the funds to displace energy from fossil fuels from the electricity grid.

I hope this clarifies things.  Both activities can be desireable, but they are very different in both cost and effect.  Basically, PBW is a way for you to seek a return on your money while also supporting companies that pursue socially desireable goals.  TerraPass is a way to directly decrease carbon emissions by a quantifiable amount.

Here's the plug (you knew it was coming): it generally only costs $30-$80 to offset a whole year's worth of emissions from your car or truck.  Learn more and calculate your own climate impact for free on our Web site.