Renewable Energy Certificates (RECs), also known as Green tags, Renewable Energy Credits, Renewable Electricity Certificates, or Tradable Renewable Certificates (TRCs), are tradable, non-tangible energy certificates in the United States that represent proof that 1 megawatt-hour (MWh) of electricity was generated from an eligible renewable energy resource (renewable electricity) and was fed into the grid. Solar renewable energy certificates (SRECs) are RECs that are specifically generated by solar energy.

There are two main markets for renewable energy certificates in the United States – compliance markets and voluntary markets.

Compliance markets are created by a policy that exists in 29 U.S. states, the District of Columbia, and Puerto Rico, called Renewable Portfolio Standard. In these states, the electric companies are required to supply a certain percent of their electricity from renewable generators by a specified year. For example, in California the law is 33% renewable by 2020, whereas New York has a 24% requirement by 2013. Electric utilities in these states demonstrate compliance with their requirements by purchasing RECs; in the California example, the electric companies would need to hold RECs equivalent to 33% of their electricity sales.

Voluntary markets are ones in which customers choose to buy renewable power out of a desire to use renewable energy. Most corporate and household purchases of renewable energy are voluntary purchases. Renewable energy generators located in states that do not have a Renewable Portfolio Standard can sell their RECs to voluntary buyers, usually at a cheaper price than compliance market RECs.

RECs can be traded directly from buyer to seller, but third party marketers, brokers, or asset managers are commonly found in the marketplace. Renewable generation facilities will often sell their credits to these entities, who then resell them on the market at a later date.

Texas developed the first comprehensive RECs system in the U.S., a web-based platform that provides for the issuance, registration, trade, and retirement of RECs. The Texas REC Program, which only tracks renewable energy certificates, started operating in July 2001.

In the Western United States, RECs are traded on the Western Renewable Energy Generation Information System (WREGIS) as part of the Western Electricity Coordinating Council (WECC). The WECC encompasses 14 states, 2 Canadian provinces, and the northern Baja Mexico.

Prices depend on many factors, such as the vintage year the RECs were generated, location of the facility, whether there is a tight supply/demand situation, whether the REC is used for RPS compliance, even the type of power created. Solar renewable energy certificates or SRECs, for example, tend to be more valuable in the 16 states that have set aside a portion of the RPS specifically for solar energy. This differentiation is intended to promote diversity in the renewable energy mix which in an undifferentiated, competitive REC market, favors the economics and scale achieved by wind farms.