Renewable energy generators sell two products, and only one of them is power.

For every megawatt of power they produce from sustainable sources, they also produce a renewable energy credit, which they can then sell on the commodities market.

Every credit can be sold exactly once.

The credits' value depends on where they come from. A credit created by a megawatt of solar power in New Jersey trades for hundreds of dollars. Other credits trade for only a couple of bucks per megawatt.

The credits are intended to bring money into the renewable market nationally. They allow customers to invest in renewable projects no matter where those projects are.

The owner of a credit gets to claim tax and public relations benefits that come with green power.

For Georgia's large customers especially, buying credits is a substantially cheaper alternative than buying power through either of the state's two green power programs.

The state's green programs, though, are intended to develop renewable resources here. In areas with abundant renewable energy sources, the credit program supports the industry. But in Georgia, struggling to develop renewable energy, it allows customers who buy credits elsewhere to claim the benefit of being green without helping build the local green power business.

Georgia Power and Green Power EMC, the green program run by the state's electric co-ops, buy both the energy and the related credits from their green suppliers.

By doing so, they ensure that the credits that make their programs green are linked to power being generated in Georgia.