I'd break this down into two questions:

• Would it be better for you to pay for a neighbor of yours to switch to the green mix or give that money to GiveWell?
• Is there something special about the energy you use coming from renewable sources as opposed to the energy someone else uses?

For the first question, subsidising renewable energy is probably a good thing, but there's no reason to expect this particular opportunity to be up there with the world's best organizations. For the second it doesn't seem to me that it matters. So I'd say buy the normal stuff and give the difference to the best organization you can find.