| VAC | Voltage Alternating Current |
| 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. |
| Vane | A large piece of material used behind the generator to hold the blades in the direction of the wind. |
| Vapor Barrier | A material with a permeance of one perm or less which provides resistance to the transmission of water vapor. |
| Vaporize | to change into the gas form anything which is normally a liquid or a solid; the term is most commonly is used in reference to water (which vaporizes to steam). |
| Variable-Speed Wind Turbines | Turbines in which the rotor speed increases and decreases with changing wind speeds. Sophisticated power control systems are required on variable speed tubines to insure that their power maintains a constant frequency compatible with the grid. |
| Varistor | A non-ohmic or voltage-dependent variable resistor. Normally used as over-voltage limiters to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground. |
| VAV System (Variable Air Volume System) | A mechanical HVAC system capable of serving multiple zones which controls the temperature maintained in a zone by controlling the amount of heated or cooled air supplied to the zone. |
| Vdc | Voltage Direct Current |
| Vented Cell | A battery designed with a vent mechanism to expel gases generated during charging. |
| Ventilation | The process of supplying or removing air by natural or mechanical means to or from any space. Such air may or may not have been conditioned or treated. |
| Vertical Integration | An arrangement whereby the same company owns all the different aspects of making, selling, and delivering a product or service. In the electric industry, it refers to the historically common arrangement whereby a utility would own its own generating plants,transmission system, and distribution lines to provide all aspects of electric service. |
| 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. |
| Visible Light Transmittance | The ratio of visible light transmitted through a substance to the total visible light incident on its surface. |
| VMP | The voltage at which a PV device is operating at maximum power. |
| VOC | Open Circuit Voltage. |
| Volcano | an opening in the Earth’s crust from which lava, steam, and/or ashes erupt (or flow), either continuously or at intervals. |
| Volt | A unit for measuring the force used to produce an electric current; the push or force that moves electric current through a conductor. |
| 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 | A measure of the force or "push" given the electrons in an electrical circuit; a measure of electrical potential. Analogy-pressure in a water pipe. AKA Potential. |
| Voltage At Maximum Power (VMP) | The voltage at which maximum power is available from a photovoltaic module. |
| Voltage Drop | Loss of voltage (electrical pressure) caused by the resistance in wire and electrical devices. Proper wire sizing will minimize voltage drop, particularly over long distances. Voltage drop is determined by four factors wire size, current (amps), voltage, and length of wire. Water analogy friction loss in pipe. |
| Voltage Of A Circuit (Electric Utility) | The electric pressure of a circuit, measured in volts. Usually a nominal rating, based on the maximum normal effective difference of potential between any two conductors of the circuit. |
| Voltage Protection | A sensing circuit on an Inverter 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. |
| Voltage Regulator | A device that controls the operating voltage of a photovoltaic array. |
| Voltage, Nominal | A way of naming a range of voltage to a standard. Example: A “12 volt nominal” system may operate in the range of 10 to 20 Volts. We call it “12 volts” for simplicity. |
| Voltage, Open Circuit | See OPEN CIRCUIT VOLTAGE |
| Voltage, Peak Power Point (VPP) | The voltage at which a photovoltaic module or array generates at the highest power (watts). A “12 volt nominal” PV module will typically have a peak power voltage of around 17 volts. A PV array-direct solar pump should reach this voltage in full sun conditions. In a higher voltage array, it will be a multiple of this voltage. |
| Voltmeter | A device for measuring the voltage difference between any two points in an electrical circuit. |
| Volumetric Wires Charge | A type of charge for using the transmission and/or distribution system that is based on the volume of electricity that is transmitted. |
