One cannot tell if an electron traveling along electric lines to your home or business was generated from renewable or fossil fuel sources. In order to classify renewable energy, regulating entities have an identification system in place. Energy is officially certified as "renewable" by the Electric Reliability Council of Texas (ERCOT). Each megawatt-hour of electricity generated by renewable resources is assigned a unique Renewable Energy Credit (REC) serialized and stored by ERCOT, serving as evidence of its renewable origin. When this energy is sold to end-use customers, the corresponding RECs are "retired" from the BTU ERCOT account, confirming that the energy delivered to you originates from renewable generation sources.