- U.S.C.—United States Code
- U3O8—triuranium octaoxide (nuclear)
- UAE—United Arab Emirates (oil)
- UCTE—Union for the Coordination of the Transport of Electricity, the power transmission system of continental Europe
- UF6—uranium hexaflouride (nuclear)
- UFLS—Under frequency load shedding (electricity)
- UHVAC—Ultra High Voltage Alternating Current (electricity)
- UHVDC—Ultra High Voltage Direct Current (electricity)
- ULCC—Ultra Large Crude Carrier (oil)
- UMTRA—Uranium Mill Tailings Radiation Control Act of 1978 (US)
- UNCCD—United Nations Convention to Combat Desertification
- UNDESA—UN Department of Economic and Social Affairs
- UNDP—United Nations Development Programme
- UNDP—United Nations Development Programme
- UNECE—United Nations Economic Commission for Europe
- UNEP—United Nations Environment Programme
- UNFCCC—United Nations Framework Convention on Climate Change
- UO2—uranium dioxide (nuclear)
- UO3—uranium trioxide (nuclear)
- UO4—uranyl peroxide (nuclear)
- UOX—uranium oxide (nuclear)
- URR—Ultimate Recoverable Resources
- USBR—United States Bureau of Reclamation
- USCE—United States Army Corps of Engineers
- US DOE—United States Department of Energy
- USGS—United States Geological Survey
- UVLS—Under voltage load shedding (electricity)
Read more about this topic: List Of Energy Abbreviations