Ultraviolet - Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.

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 (UPS) - The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.

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.

V

Vacuum Evaporation - The deposition of thin films of semiconductor material by the evaporation of elemental sources in a vacuum.

Vacuum Zero - The energy of an electron at rest in empty space; used as a reference level in energy band diagrams.

Valence Band - The highest energy band in a semiconductor that can be filled with electrons.
Valence Level Energy/Valence State - Energy content of an electron in orbit about an atomic nucleus. Also called bound state.

Varistor - A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground

Vented Cell - A battery designed with a vent mechanism to expel gases generated during charging.

Vertical Multijunction (VMJ) Cell - A compound cell made of different semiconductor materials in layers, one above the other. Sunlight entering the top passes through successive cell barriers, each of which converts a separate portion of the spectrum into electricity, thus achieving greater total conversion efficiency of the incident light. Also called a multiple junction cell. See multijunction device and split-spectrum cell.

Volt (V) - A unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.

Voltage - The amount of electromotive force, measured in volts, that exists between two points.

Voltage at Maximum Power (Vmp) - The voltage at which maximum power is available from a photovoltaic module.

Voltage Protection - Many inverters have sensing circuits that will disconnect the unit from the battery if input voltage limits are exceeded.

Voltage Regulation - This indicates the variability in the output voltage. Some loads will not tolerate voltage variations greater than a few percent.