A renewable energy certificate (REC) is a record that a megawatt-hour of electricity was generated from a renewable source. RECs can generally be used to comply with statewide renewable and alternative portfolio standards: A utility that has not generated enough renewable electricity to meet its target can purchase RECs from a utility that has exceeded its target. RECs can also be purchased to promote sustainability, for example if a municipal government wants to demonstrate that it is supporting renewable energy generators when none are available locally.
Tracking systems manage the creation, registration, use, and retirement of RECs. Tracking systems prevent RECs from being double-counted or used to meet renewable energy standards in more than one region. Several regions and individual states have REC tracking systems, and RECs may be traded or accepted in other regions.
WREGIS: Western Renewable Energy Generation Information System 
M-RETS: Midwest Renewable Energy Tracking System 
ERCOT: Electric Reliability Council of Texas 
PJM GATS: PJM Generation Attribute Tracking System 
NEPOOL GIS: New England Power Pool Generation Information System 
MIRECS: Michigan Renewable Energy Certification System 
NC-RETS: North Carolina Renewable Energy Tracking System 
NYSERDA: New York State Energy Research and Development Authority  (Under development)
NVTREC: Nevada Tracks Renewable Energy Credits 
NAR: North American Renewables Registry 
U.S. States & Regions: Renewable & Alternative Energy Portfolio Standards 
Read Race to the Top: The Expanding Role of U.S. State Renewable Portfolio Standards , by Barry Rabe (June 2006).