Buying RECs is like using green power
One of the largest sources of greenhouse gas emissions is electricity generation. By choosing to buy green power from your local utility, you can lower your greenhouse gas emissions dramatically.
However, if green power is not yet available in your area, you can still offset your electricity use by purchasing what are known as "renewable-energy certificates" (RECs). Also known as "green tags" or "tradable renewable certificates", RECs represent the environmental attributes associated with renewable electricity such as wind and solar. They are typically sold in kilowatt-hours or megawatt-hours. While RECs are not electricity per se, they have the same environmental effect as buying green electricity.
Here's how they work. For every unit of electricity generated from a renewable-energy project, a corresponding REC can be sold. While utility customers cannot dictate which electrons they receive (since electricity from both conventional and renewable-energy projects are mixed on the grid), they can choose where their money goes. By purchasing RECs, customers can choose to support clean, renewable energy. The money generated by the sale of RECs goes to new and existing renewable-energy facilities, thus providing a financial incentive for developers to build more renewable-energy projects.