In essence, these companies keep the renewable energy credits (REC's) and sell them on the open market for a tidy profit. Sometimes they sell them directly to power providers in other states that are trying to meet their respective renewable energy mandates.
Don't get us wrong. You're still helping to reduce the total amount of pollution in the atmosphere. And you also dosave money – sometimes a lot of money. But as Severin Borenstein of UC Berkeley points out,
"If you've installed solar at your home and are now basking in the I'm-saving-the-planet warm glow, you may be in for a splash of ice water. There's a good chance someone else has purchased your halo and is wearing it right now."