Alameda's electricity is about 85% carbon neutral; that is, it's from geothermal and hydroelectric. AP&T is actively anti-solar (I guess PV panels on top of Victorians constitutes a period architecture faux pas?) so what's an Alamedan to do, to become carbon neutral?
Well, I added up my total kWh, took 15%, and then added in the roughly 350 gallons of fuel I buy for driving the van (when I'm not driving the electric car, silly!). It works out to about 6,000 pounds of CO2 my family and I generate each year. A Green Certificate from 3 Phases Energy to cover this is only $96!
A Green Certificate is a voluntary purchase; I still have to pay my regular electric bill. The Green Certificate buys electricity to go into the grid. Here's a little science: the grid is like one big, taut water balloon. Energy producers push electrons into the grid, and consumers use those electrons within 1/10th of a second. An electron from nuclear is identical to an electron from bio-mass or an electron from a PV panel. The electrons in my lightbulb are just the ones that happen to be there at the time I use them-- I don't have any control, at my lightbulb, where those electrons came from. I can control what electrons go into the grid, though. My green power purchase pushes 400 kWh into the grid. In an ideal situation, that's 400 kWh that doesn't get generated by oil, natural gas, coal, or nuclear.
So when I buy a $96 green certificate, I provide enough energy into the grid for my household and business become carbon neutral consumers.