| UA | A measure of the amount of heat that would be transferred through a given surface or enclosure (such as a building envelope) with a one degree Fahrenheit temperature difference between the two sides. The UA is calculated by multiplying the U-Value by the area of the surface (or surfaces). |
| UDC | Utility distribution company. An entity that owns a distribution system for the delivery of energy to and from the ISO-controlled grid, and that provides regulated, retail service to eligible end-use customers who are not yet eligible for direct access, or who choose not to arrange services through another retailer. |
| Ultrahigh Voltage Transmission | Transporting electricity over bulk-power lines at voltages greater than 800 kilovolts. |
| Ultraviolet | Electromagnetic radiation in the wavelength range of 4 to 400 nanometers. |
| Unbundling | Disaggregating electric utility service into its basic components and offering each component separately for sale with separate rates for each component. For example, generation, transmission and distribution could be unbundled and offered as discrete services. |
| Unconditioned Space | A space that is neither directly nor indirectly conditioned space, which can be isolated from conditioned space by partitions and/or closeable doors. |
| Underground Feeder (Uf) | May be used for photovoltaic array wiring if sunlight resistant coating is specified; can be used for interconnecting balance-of-system components but not recommended for use within battery enclosures. |
| Underground Service Entrance (Use) | May be used within battery enclosures and for interconnecting balance-of-systems. |
| Uninterruptible Power Supply | A power supply capable of providing continuous no-stop power, usually by utilizing batteries. |
| Uninterruptible Power Supply (UPS) | A power supply capable of providing continuous uninterruptible service; normally containing batteries to provide energy storage. |
| Universal Service | Electric service sufficient for basic needs (an evolving bundle of basic services) available to virtually all members of the population regardless of income. |
| Unleaded Gasoline | Gasoline that has had tetraethyl lead removed in conformance with federal and state regulations. |
| Upgrade (Electric Utility) | Replacement or addition of electrical equipment resulting in increased generation or transmission capability. |
| Uprate (Electric Utility) | An increase in the rating or stated measure of generation or transfer capability. |
| Upstream | A term used in the petroleum industry referring to the exploration and production side of the business. This includes pipelines but production before reaching the refinery. |
| Upwind | In relation to a wind turbine, toward the wind. An upwind turbine has its blades on the upwind side of the tower. |
| Upwind Generator | A wind generator which its propellers faces into the wind (vane behind) |
| Uranium | A radioactive element, found in ores, of which atoms can be split to create energy. |
| Uranium Enrichment | The process of increasing the percentage of pure uranium above the levels found in naturally occurring uranium ore, so that it may be used as fuel. |
| Utility | A regulated entity which exhibits the characteristics of a natural monopoly. For the purposes of electric industry restructuring, "utility" refers to the regulated, vertically-integrated electric company. "Transmission utility" refers to the regulated owner/operator of the transmission system only. "Distribution utility" refers to the regulated owner/operator of the distribution system which serves retail customers. |
| Utility Grid | Also see "Power Grid." A common term referring to an electricity transmission and distribution system. |
| Utility-Interactive Inverter | An inverter that can function only when tied to the utility grid, and uses the prevailing line-voltage frequency on the utility line as a control parameter to ensure that the photovoltaic system’s output is fully synchronized with the utility power. |
| Utility-Intertie (Ui) System | See GRID-TIE SYSTEM. |
| U-Value Or U-Factor | A measure of how well heat is transferred by the entire window |
