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Alternating Current (AC)
Electric current which repeatedly changes polarity from negative to positive and back again. The most commonly used form of alternating current does so in a sine wave pattern.

Amperage (Ampere, Amp, A)
A unit of electrical current (I). The relationship between power, current and voltage is expressed in the Power Equation, P=IV. The relationship between voltage, current, and resistance is expressed in Ohm's Law, V=IR.

Amorphous Silicon
A non-crystalline semiconductor material. Easier and cheaper to make than crystalline, but less efficient and slowly degrades over time. Also called thin film.

Array
PV modules wired together in series and parallel to produce desired electrical characteristics. PV arrays connected to a single metering point constitute a system.

Azimuth
The orientation of a PV array relative to direction, in degrees, in which 180 refers to due South, 90 refers to due East, and 270 refers to due West.

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Balance of System (BOS)
Represents all components and costs other than the PV modules. It includes design costs, land, site preparation, system installation, support structures, power conditioning, operation and maintenance costs, batteries, indirect storage, and related costs.

British Thermal Unit (BTU)
The amount of heat energy required to raise the temperature of one pound of water from 60 degrees F to 61 degrees F at one atmosphere pressure.

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Capacity
The maximum amount of power a system or its component parts is expected to be capable of producing over a short time period.

Charge Controller
A component of a photovoltaic system that controls the flow of current to and from the battery subsystem to protect batteries from overcharge, overdischarge, or other control functions. The charge controller may also monitor system operational status.

Current (I)
The flow of charge through an electrical conductor, usually measured in Amperes (A). The relationship between power, current and voltage is expressed in the Power Equation, P=IV. The relationship between voltage, current, and resistance is expressed in Ohm's Law, V=IR. In fluid systems current is analogous to the flow rate (measured, for example, in gallons per minute).

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Datalogger
A device that records and transmits data sampled by sensors.

Direct Current (DC)
A one-way flow of charge through an electrical conductor. Current is caused by a flow of negatively charged electrons along a metallic conductor.

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Electric Circuit
Path followed by electrons from a power source (generator or battery) through an external line (including devices that use the electricity) and returning through another line to the source.

Electric Current
A flow of electrons; electricity, amps.

Electrical Grid
An integrated system of electricity distribution, usually covering a large area. As in "off the grid".

Energy
Usable heat or power. The amount (or average amount) of power produced, consumed, or delivered over a specified time period. Typically measured in watt-hours (Wh), kilowatt-hours (kWh), or megawatt-hours (MWh).

Ethylene Vinyl Acetate (EVA)
An encapsulant used between the glass cover and the solar cells in PV modules. It is durable, transparent, resistant to corrosion, and flame retardant.

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Facility
The physical location of a system.

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Generation
The electricity generated by a system as recorded by a KWH meter, recorded in KWH or MWH.

Grid-Connected (PV system)
A PV system in which the PV array acts like a central generating plant, supplying power to the grid.

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Hot Spot
An undesirable phenomenon of PV device operation whereby one or more cells within a PV module or array act as a resistive load, resulting in local overheating or melting of the cell(s).

Hybrid System
A PV system that includes other sources of electricity generation, such as wind or fossil fuel generators.

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Incident Light
Light that shines onto the face of a solar cell or module.

Insolation
Sunlight, direct or diffuse; from 'incident solar radiation.' Not to be confused with 'insulation.' Equal to about 1000 watts per square meter at high noon.

Inverter
A device that converts DC electricity into AC electricity.

Inverter KWac (KWac)
A rating of the capacity of a system's inverter(s). The inverter capacity should be as large or larger than the System KWac, but is sometimes smaller than the System KWdc.

Inverter Wac
The rated AC capacity of an inverter, in Watts. Inverters are typically around 90% efficient, meaning they convert about 90% of DC input into AC electricity.

Irradiance
See Solar Radiation.

I-V curve
A graphical presentation of the current versus the voltage from a photovoltaic device as the load is increased from the short circuit (no load) condition to the open circuit (maximum voltage) condition. The shape of the curve characterized cell performance.

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Joule
A joule is the energy required to push with a force of one Newton for one meter.

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Kilowatt (kW)
A measure of instantaneous electric power consumption or production. 1kW = 1000 Watts

Kilowatt-Hour (kWH)
The amount of energy consumed (produced) by a continuous load (generation) of 1 kilowatt over one hour. Equal to one thousand watt-hours.

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Load
Anything in an electrical circuit that, when the circuit is turned on, draws power from that circuit.

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Maximum Power Point (MPP)
The point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum.

Maximum Power Point Tracker (MPPT)
A power conditioning unit that automatically operates the PV-generator at its MPP under all conditions. An MPPT will typically increase power delivered to the system by 10% to 40%, depending on climate conditions and battery state of charge.

Megawatt (MW)
A measure of instantaneous electric power consumption or production. Equal to one thousand KW.

Megawatt-Hour (MWH)
The amount of energy consumed (produced) by a continuous load (generation) of 1 megawatt over one hour. Equal to one thousand KWH.

Module
A group of PV cells wired in series and parallel to achieve desired electrical characteristics and packaged as a single unit. Modules are wired together in series and parallel to make arrays.

Module Wdc
The rated DC capacity of each PV module at standard test conditions.

Monolithic
Fabricated as a single structure.

Multi-crystalline
Material that is solidified at such as rate that many small crystals (crystallites) form. The atoms within a single crystallite are symmetrically arranged, whereas crystallites are jumbled together. These numerous grain boundaries reduce the device efficiency. A material composed of variously oriented, small individual crystals. (Sometimes referred to as polycrystalline or semi-crystalline).

Multi-junction Device
A photovoltaic device containing two or more cell junctions, each of which is optimized for a particular part of the solar spectrum, to achieve greater overall efficiency.

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Nominal Operating Cell Temperature (NOCT)
The PV cell temperature under standard operating conditions. Under most climatic conditions, PV cells in a module operate hotter than the ambient air temperature. The NOCT is an indicator of the temperature differential between PV cells and air temperature.

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Ohm (R)
A unit of electrical resistance. The relationship between voltage, current and resistance is governed by Ohm's Law, V=IR.

Ohm's Law (V=IR)
An equation that expresses the relationship between voltage (V), current (I) and resistance (R).

Open-Circuit Voltage (Voc)
The voltage across a PV cell in full sunlight when there is no current flowing; the highest possible voltage.

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Parallel Connection
A way of joining two or more electricity-producing devices (i.e., PV cells or modules) by connecting positive leads together and negative leads together; such a configuration increases the current.

Peak Load; Peak Demand
The maximum load, or usage, of electrical power occurring in a given period of time, typically a day.

Peak/Rated kWac
The ratio of the peak kWac recorded divided by the the system's rated kWac. An indicator of system performance, as systems can generally be expected to operate near their peak at some point during. Low values do not always indicate a problem, as they may be due to poor seasonal sunlight conditions (e.g. near-horizontal arrays in northern latitudes during winter).

Photovoltaic (PV)
Literally, "light" (photo) and "electricity" (voltaic). The class of equipment used to generate electricity directly from sunlight.

Photovoltaic (PV) Array
An interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure with a common support or mounting. In smaller systems, an array can consist of two modules plus a support structure or mounting.

Photovoltaic (PV) Cell
A device that converts light directly into electricity. The basic building block of a PV system . Individual PV cells are wired together in series and parallel to make modules.

Photovoltaic (PV) Efficiency
The ratio of electric power produced by a cell at any instant to the power of the sunlight striking the cell. This is typically about 9% to 14% for commercially available cells.

Photovoltaic (PV) Module
A number of PV cells electrically interconnected (in either series or parallel) and mounted together, usually in a sealed unit of convenient size to make shipping, handling and assembly into arrays easier.

Photovoltaic (PV) Panel
Often used interchangeably with PV module (especially in one-module systems), but more accurately used to refer to a physically connected collection of modules.

Photovoltaic (PV) System
A complete set of components for converting sunlight into electricity by the PV process, including the array and balance of system components.

Plane of Array
Refers to the orientation of a sensor - usually a pyranometer - to the same azimuth and tilt as a PV array in order to accurately measure the solar radiation actually available to the array.

Polycrystalline
See 'Multi-crystalline.'

Potential
The difference in electrical potential between two points in an electric circuit; measured in volts and expressed as the voltage (V). The relationship between power, current and voltage is expressed in the Power Equation, P=IV. The relationship between voltage, current, and resistance is expressed in Ohm's Law, V=IR.

Power
The rate at which energy is produced, typically expressed in watts (W), kilowatts (kW), or megawatts (MW). The relationship between power, current and voltage is expressed in the Power Equation, P=IV.

Power Equation (P=IV)
The equation expressing the relationship between power (P, in watts), current (I, in amps), and voltage (V, in volts). For example, a 20 amp breaker on a 110 volt circuit is designed to trip when loads on the circuit exceed 2,200 watts (2,200 watts = 20 amps * 110 volts).

PV
Abbreviation for photovoltaic(s).

Pyranometer
A sensor that measures solar radiation.

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Remote Systems
Photovoltaic systems off of the utility grid.

Renewable Energy Credit (REC)
A trade-able certificate representing the generation attributes of energy derived from a qualified renewable energy source. In the U.S., formal markets for RECs are established in New England and Texas, and are developing elsewhere. Informal and voluntary markets are active or emerging in several other U.S. regions. RECs are also called renewable energy certificates, tradable renewable certificates (TRCs), "green tags", and other names.

Resistance (R)
The ratio of the potential difference (i.e., voltage) across an electrical component (i.e., a resistor) to the current passing through it. The relationship between voltage, current and resistance is described by Ohm's Law, V=IR.

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Semiconductor
Any material that has a limited capacity for conducting an electric current. Generally falls between a metal and an insulator in conductivity. Certain semiconductors, including silicon, gallium arsenide, copper indium diselenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process.

Sensor
A device that translates a condition (temperature, sunlight) or event (kWh production) into electronic signals interpreted by a datalogger.

Series Connection
A way of joining photovoltaic cells or batteries by connecting positive leads to negative leads; such a configuration increases the voltage.

Short-Circuit Current (Isc)
The current flowing freely from a PV cell through an external circuit that has no load or resistance; the highest current possible.

Silicon (Si)
A chemical element, atomic number 14, semi-metallic in nature, dark gray, an excellent semiconductor material. A common constituent of sand and quartz (as the oxide). Crystallizes in face-centered cubic lattice like a diamond. The most common semiconductor material used in making photovoltaic devices.

Single-Crystal Material
A material that is composed of a single crystal or a few large crystals.

Solar Cell
See 'Photovoltaic cell.'

Solar Energy
Energy from the sun.

Solar Radiation
Radiant energy deriving from the sun. Measured in watts per square meter.

Stand-alone (PV system)
A photovoltaic system not connected to a main electric grid. May be solar-only or hybrid. May or may not have storage batteries, but most stand-alone systems require batteries or some other form of storage (e.g. water reservoir for pumping).

Stand-off Mounting
Technique for mounting a PV array on a sloped roof, which involves mounting the modules a short distance above the pitched roof and tilting them to the optimum angle.

Standard Operating Conditions (SOC)
A standard set of conditions used for comparing PV module performance. The conditions are: ambient air temperature of 20ºC, solar irradiation of 800 W/m2, and wind speed of 1 m/s.

Standard Test Conditions (STC)
A standard set of conditions used for comparing PV module performance. The conditions are: solar irradiation of 1,000 W/m2 and cell temperature of 25ºC. Abbreviated as "STC", as in "Module Rating (DCstc)".

Substrate
The physical material upon which a photovoltaic cell is made.

Superstrate
The covering on the sun side of a PV module, providing protection for the PV materials from impact and environmental degradation while allowing maximum transmission of the appropriate wavelengths of the solar spectrum.

System
All equipment used to generate electricity whose output is metered at a single point.

System KWac (KWac)
A rating of the AC electricity generating capacity of a system. This approximates the maximum expected AC output (peaking power) of a PV system under ideal conditions. Although there are many ways to calculate an AC system rating, for simplicity and consistency we multiply the System KWdc (the sum of the DCstc ratings of all the PV modules in a system) by 0.90 to obtain System KWac. This method is based on estimated wire and mismatch losses of about 4% and inverter losses of about 6%. Other factors, such as high temperatures, could also degrade the actual system KWac. For example, modules are rated at 20 degrees C, but experience a degradation of about 0.5% for every degree C above 20. In hot climates, modules can operate at very high temperatures and the effect of this temperature degradation can be quite high.

System KWdc
A rating of the DC electricity generating capacity of a system. In the case of PV systems, it is the sum of the rated module DC capacity (module Wdc) of all modules deployed in the system.

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Thick-crystalline Materials
Semiconductor material, typically measuring from 200-400 microns thick, that is cut from ingots or ribbons. same as 'Bulk-crystalline Materials.'

Thin-film
A layer of semiconductor material, such as copper indium diselenide, cadmium telluride, gallium arsenide, or amorphous silicon, a few microns or less in thickness, used to make photovoltaic cells. Commonly called amorphous.

Tilt
The inclination of a PV array in the direction of the azimuth, in degrees, in which 0 refers to horizontal and 90 refers to vertical.

Total kWh
The sum of the Locked kWh and Adjust kWh columns.

Tracking Array
PV array that follows the path of the sun to maximize the solar radiation incident on the PV surface. The two most common orientations are (1) one axis where the array tracks the sun east to west and (2) two-axis tracking where the array points directly at the sun at all times. Tracking arrays use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy. Typically, a single axis tracker will give you 15% to 25% more power per day, and dual axis tracking will add about 5% to that. Depends somewhat on latitude and
season.

Transformer
Steps AC voltage up or down, depending on the application.

Two-axis Tracking
A system capable of rotating independently about two axes (e.g., vertical and horizontal) and following the sun for maximum efficiency of the solar array.

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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 PV system's output is fully synchronized with the utility power.

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VAC
Volts ac

VDC
Volts dc

Vmp
Voltage at maximum power

Voc
Open-circuit voltage

Voltage (Volt, V)
A unit of electrical potential difference between two points. The relationship between power, current and voltage is expressed in the Power Equation, P=IV. The relationship between voltage, current, and resistance is expressed in Ohm's Law, V=IR.

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Wafer
A thin sheet of semiconductor material made by mechanically sawing it from a single-crystal or multi-crystal ingot or casting.

Watt (W)
The unit of electric power, or amount of work (J), done in a unit of time. One ampere of current flowing at a potential of one volt produces one watt of power.

Watt-hour (Wh)
See 'Kilowatt-hour.

Watt-hour (Wh)
A quantity of energy. One watt-hour of electricity is consumed when one watt of power is used for one hour.