There's a REC market, and "production" of those certificates is tied 1:1 to input into the grid, which is metered and monitored by the applicable interconnect (

There's a REC market, and "production" of those certificates is tied 1:1 to input into the grid, which is metered and monitored by the applicable interconnect (as a necessity for its operation). When you purchase these renewable energy credits you're essentially bidding up a bonus for producers of the appropriate energy, above and beyond the price they got for the electricity itself. Google has entered some long-term contracts for the acquisition of these credits from renewable energy providers, which in the short term bids up the price of credits for everyone else and in the longer term leads to more and larger renewable energy producers. Also note that Google isn't doing this entirely via the credits markets — it's also reducing its usage and where applicable is working with energy providers to create nearby generation.