A national Clean Energy Standard (CES) would require that some percentage of total energy supply comes from "clean" energy resources. The types of energy resources that are considered "clean" depend on the definition in the CES. Many CES proposals have included energy efficiency as a clean energy option. Energy efficiency is generally less expensive than other sources of clean energy and including it in a national CES would lower the cost of implementing the standard. This fact sheet provides additional detail about the opportunities for energy efficiency in a national CES.