Activated shelf life: The time it takes for the capacity of a charged battery to fall to an unusable level when stored at a specified temperature. 
Activation voltage: The voltage at which the controller will operate to protect the batteries.
AGM (Adsorbed Glass Mat): a newer type of battery construction that uses saturated adsorbant glass mats rather than gelled or liquid electrolyte. AGM batteries are typically more expensive than flooded (liquid), but offer enhanced reliability.
Air mass: The air mass relates to the path length of solar radiation through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere thickness. Approximately equal to the secant of the zenith angle, i.e. the angle from directly overhead to a line to the sun. 
Alternating current (AC): Electrical current that continually reverses direction of flow. The frequency at which it reverses is measured in cycles-per-second, or Hertz (Hz). The magnitude of the current itself is measured in amps (A).
Alternator: A device for producing Alternating Current ("AC") electricity. Usually driven by a motor, but can also be driven by other means, including water and wind power.
Ambient temperature: The temperature of the surroundings.
Ammeter: A device used for measuring current flow at any point in an electrical circuit.
Amorphous silicon: A thin-film solar PV cell material which has a glassy rather than crystalline structure. Made by depositing layers of doped silicon on a substrate normally using plasma-enhanced chemical vapor deposition of silane.
Amp hour: The quantity of electrical energy corresponding to the flow of current of one ampere for one hour. The term is used to quantify the energy stored in a battery. Most batteries are rated in Ah.
Ampere (A) or amp: The unit for the electric current; the flow of electrons. One amp is 1 coulomb passing in one second. One amp is produced by an electric force of 1 volt acting across a resistance of 1 ohm.
Anemometer: A device used to measure wind speed.
Angle of incidence: Angle between the normal to a surface and the direction of incident radiation; applies to the aperture plane of a solar panel. Only minor reductions in power output within plus/minus 15 degrees.
Anode: The positive electrode in a battery.  The positive terminal of a diode.
Anti-reflection coating: A thin coating of a material with a specific refractive index applied to a cell to reduce the reflection of light.
Array: A number of solar modules connected together in a single structure.
Array current: The electrical current output of a PV array when exposed to sunlight.
Array operating voltage: The voltage output of a PV array when exposed to sunlight and feeding a load.
Autonomous system: A PV System that operates without any other energy generating source.
Availability: The quality or condition of a PV system that is available to provide power to a load. Usually measured in hours per year.
AWG: American Wire Gauge: a standard system for designating the size of electrical wire. The higher the number, the smaller the wire. Most house wiring is #12 or 14.
Azimuth: The Angle between the north direction and the projection of the surface normal into the horizontal plane; measured clockwise from north. As applied to the PV array, 180 degree azimuth means the array faces due south.

B

Balance of system (BOS): All the parts of a PV System excluding the solar module
Ballast: a circuit used to stabilize an electric current, for example, in a fluorescent light.
Battery: A system in which stored chemical energy is converted directly into electrical energy.  Can be either rechargeable or non-rechargeable. Different to a fuel cell in that it contains a fixed quantity of stored chemical energy rather than a continuous supply of fuel.
Battery capacity: The total number of ampere-hours (Ah) that a fully charged battery can output.
Battery cell: An individual unit of a battery that can store electrical energy and is capable of furnishing a current to an external load. For lead-acid batteries the voltage of a cell (fully charged) is about 2.2 volts dc. A battery may consist of a number of cells.
Battery charger: A device used to charge a battery by converting (usually) mains voltage AC to a DC voltage suitable for the battery. Chargers often incorporate some form of regulator to prevent overcharging and damage to the battery.
Battery cycle Life: The number of times a battery can undergo a cycle of discharge and recharge before failing. Cycle Life is normally specified as a function of discharge rate and temperature.
Battery self-discharge: Energy loss by a battery that is not under load.
Battery state of charge (SOC): Extent of battery charge status as a percentage of full charge.  Also 100 per cent minus the Depth of Discharge.
BIPV: Building Integrated Photovoltaics. As the name suggests, this is where PV modules are integrated in to the building construction materials as on integrated unit.
Blocking diode: A diode used to prevent current flow in an undesirable direction e.g. from the rest of the PV array to a failed module or from the battery to the PV array when current generation is low.
British Thermal Unit (BTU): The amount of heat energy required to raise one pound of water from a temperature of 60 degrees F to 61 degrees F at one atmosphere pressure. One Watt hour equals 3,413 BTU.
Bypass diode: A diode connected across one or more solar cells in a photovoltaic module such that the diode will conduct if the cell(s) become reverse biased. [UL 1703]

C

Capacitor: An electronic component used for the temporary storage of electricity, as well for removing unwanted noise in circuits. A capacitor will block Direct Current but will pass Alternating Current.
Capacity: See Battery Capacity. 
Captive electrolyte battery: A battery with an immobilized electrolyte (gelled or absorbed in a material).
Cathode: The negative electrode in an electrochemical cell. Also, the negative terminal of a diode.
Cathodic protection: A method of preventing oxidation (rusting) of exposed metal structures, such as bridges and pipelines, by imposing between the structure and the ground a small electrical voltage that opposes the flow of electrons and that is greater than the voltage present during oxidation.
Cell efficiency: The ratio of the electrical energy produced by a photovoltaic cell (under full sun conditions or 1 kW/m2) to the energy from sunlight falling upon the photovoltaic cell.
Cell junction: The area of immediate contact between two layers (positive and negative) of a photovoltaic cell. The junction lies at the center of the cell barrier or depletion zone.
Cell: The basic unit of a PV module or battery.  The most basic unit that contains the necessary materials, such as electrodes and electrolyte in a battery, to produce electricity. 
Charge: The process of inputting electrical energy to a battery.
Charge controller: A component that controls the flow of current to and from the battery subsystem to protect the batteries from overcharge and over discharge. Essential for ensuring that batteries obtain maximum state of charge and longest life. The charge controller may also monitor system performance and provide system protection. Charge Controllers are also sometimes called Regulators.
Charge factor: A number corresponding to the time (in hours) for which a battery can be charged at a constant current without damaging it. Usually expressed as a function of battery capacity, e.g. C/10 indicates a charge factor of 10 hours. Related to Charge Rate.
Charge rate: A measure of the current used to charge a battery as a proportion of its capacity.
Circuit: A continuous system of conductors providing a path for electricity
Circuit breaker: A circuit breaker acts like an automatic switch that can shut the power off when it senses too much current.
Cloud enhancement: The increase in solar intensity due to reflected light from nearby clouds.
Cogeneration: The joint production of electricity and useful heat at a single facility, resulting in more efficient use of the thermal energy.
Concentrator: A photovoltaic device that uses optical elements (e.g. mirrors or lenses) to increase the amount of light incident on a solar PV cell. Concentrator arrays track the sun and use only direct sunlight since the diffuse portion cannot be focused.  Concentrators therefore work best in clear sky locations.  Efficiency is increased, but cell life may be reduced because operating temperatures are higher.
Conductor: A material used to transfer, or conduct, electricity, often in the form of wires.
Conduit: A pipe or elongated box used to house and protect electrical cables.
Conversion efficiency: The ratio of the electrical energy generated by a solar PV cell to the solar energy impacting the cell.
Cross-flow turbine: A turbine where the flow of water is at right angles to the axis of rotation of the turbine. Crystalline silicon: A type of PV cell material made from a single crystal or polycrystalline ingot of silicon.
Current: Current is the flow of electric charge in a conductor between two points having a difference in electrical potential (voltage) and is measured in Amps.
Current at maximum power (Imp): The current at which maximum power is available from a module. [UL 1703]
Cut-off voltage: The voltage levels at which the charge controller (regulator) disconnects the PV array from the battery, or the load from the battery.
Cycle: The discharge and re-charge of a battery, one complete charge/discharge cycle of the battery.
Cycle life: Number of charge-discharge cycles a battery can perform under specified conditions before it fails to meet its specified performance (e.g. capacity decreases to 80% of nominal capacity).

D

Days of storage: The number of days that a stand-alone system will power a specified load without solar energy input.  A measure of system autonomy.
DC to DC converter: Electronic circuit to convert dc voltages (e.g., PV module voltage) into other levels (e.g., load voltage). Can be part of a maximum power point tracker (MPPT).
Dealer: A Retailer of PV products and/or PV Systems
Deep cycle battery: A battery designed to regularly discharge 80% of its capacity before recharging.
Deep discharge: Discharging a battery by more than 80% of its full charge.
Depth of discharge (DOD): the amount of energy withdrawn from a battery or cell expressed as a percentage of its rated capacity.
Design month: The month in which the combination of insolation and load requires the maximum energy from the array.
Diffuse insolation: Incident sunlight received indirectly because of scattering due to clouds, fog, particulates, or other obstructions in the atmosphere. The other component of sunlight is Direct.
Diffuse radiation: Radiation received from the sun after reflection and scattering by the clouds, fog, haze, dust or other substances in the atmosphere, and the ground.
Diode: Electronic device that allows current flow only in one direction.
Direct beam radiation: Radiation received by direct sunlight. Measured by a pyrheliometer with a solar aperture of 5.7° to transcribe the solar disc.
Direct current (DC): Electrical current that flows only in one direction, although it may vary in magnitude. Contrasts with alternating current.
Direct insolation: Sunlight falling directly upon a collector. Opposite of diffuse insolation.
Direct radiation: Light that has traveled in a straight path from the sun (also referred to as beam radiation). An object in the path of direct radiation casts a shadow on a clear day.
Discharge: Withdrawal of electrical energy from a battery.
Discharge factor: A number equivalent to the time in hours during which a battery is discharged at constant current usually expressed as a percentage of the total battery capacity, i.e., C/5 indicates a discharge factor of 5 hours.
Discharge rate: A measure of the current withdrawn from a battery over time, expressed as a percentage of battery capacity. A C/5 discharge rate indicates a current of one-fifth of the rated capacity of the battery.
Disconnect: Switch used to connect or disconnect components in a PV system.
Dispatchability: The ability of a generating unit or other source of electric power to vary output.
Dispatchable power: Energy output that can be planned on and typically provides a continuous power output. Solar power and Wind power in not dispatchable without configuration with out some other power or storage mechanism. Hydrocarbon based power plants or nuclear plants are dispatchable.
Distributed systems: Systems that are installed at or near the location where the electricity is used, as opposed to central systems that supply electricity to grids. A residential photovoltaic system is a distributed system.
Distributor: Using means a wholesaler of PV products
Downtime: Time when the PV system cannot provide power to the load, expressed either in hours per year or as a percentage.
Dry cell battery: A battery that uses a solid paste for an electrolyte.
Duty cycle: The ratio of active to total time, used to describe the operating regime of loads in PV systems.
Duty rating: The amount of time an inverter can operate at full rated power. Some inverters can operate at their rated power for only a short time without overheating.

E

Earth: Refers to physically connecting a part of an electrical system to the ground, done as a safety measure, by means of a conductor embedded in suitable soil.
Earth-leakage circuit breaker (ELCB): A device used to prevent electrical shock hazards in mains voltage power systems, including independent power systems. Also known as residual current devices (RCD's).
Efficiency: The ratio of output power or energy to input power or energy, expressed as a percentage.
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: The flow of electrons measured in Amps.
Electrical grid: A network for electricity distribution across a large area.
Electricity: The movement of electrons (a sub-atomic particle), produced by a voltage, through a conductor.
Electrode: An electrically conductive material, forming part of an electrical device, often used to lead current into or out of a liquid or gas. In a battery, the electrodes are also known as plates.
Electrolysis: A chemical reaction caused by the passage of electricity from on electrode to another.
Electrolyte: The medium that provides ionic transport between the electrodes of a battery. All common batteries contain an electrolyte, such as the sulfuric acid used in lead-acid batteries.
Energy density: The ratio of the energy available from an energy storage device such as a battery to its volume (Wh/m3) or weight (Wh/kg).
Energy payback time: The time required for any energy producing system or device to produce as much energy as was required in its manufacture. For solar electric panels, this is normally in the range 6-36 months.
Energy: Power consumed multiplied by the duration of use. For example, 1000 Watts used for four hours is 4000 Watt hours.
Equalization charge: Periodical overcharging the batteries for a short time to mix the electrolyte solution in batteries.
EVA (Ethylene Vinyl Acetate): An encapsulant used between the glass cover and the solar cells in PV modules. It is durable, transparent, resistant to corrosion, and flame retardant.

F

Fill factor: On an I-V (current-voltage) curve characterizing the output of a solar cell or module, the ratio of the maximum power to the product of the open-circuit voltage and the short-circuit current. The higher the fill factor (FF) the "squarer" the shape of the I-V curve.
Fixed tilt array: A solar PV array set at a fixed angle to the horizontal.
Flat-plate PV: A solar PV array or module that does not contain concentrating devices and so responds to both direct and diffuse sunlight.
Float charge: A battery charge current that is equal to, or slightly greater than, the self-discharge rate.
Float life: The time (usually in years) a battery can maintain its stated capacity when kept at float charge.
Flooded cell battery: A form of rechargeable battery where the plates are completely immersed in a liquid electrolyte. Most cars use flooded-cell batteries. Flooded cell batteries are the most commonly used type for independent and remote area power supplies.
Fluorescent light: A form of lighting that uses long thin tubes of glass which contain mercury vapor and various phosphor powders (chemicals based on phosphorus) to produce white light. Generally considered to be the most efficient form of home lighting.
Frequency: The number of cycles or repetitions per unit time of a complete waveform, in electrical applications usually expressed in cycles per second or Hertz (Hz). Electrical equipment in the United States requires 60 Hz, in Europe 50Hz. 
Fresnel lens: A concentrating lens, positioned above and concave to a PV material to concentrate light on the material.
Fuel cell: An electrochemical device that converts the energy of a fuel directly into electricity and heat and is therefore very energy efficient.
Fuse: A fuse is a device used to protect electrical equipment from short circuits. Fuses are made with metals that are designed to melt, when the current passing through them is high enough. When the fuse melts, the electrical connection is broken, interrupting power to the circuit or device.

G

Gassing: Gaseous by-products when charging a battery, e.g. hydrogen from a lead acid battery.
Gel-type battery: Lead-acid battery in which the electrolyte is immobilized in a gel. Usually used for mobile installations and when batteries will be subject to high levels of shock or vibration.
Generator: A mechanical device used to produce DC electricity. Power is produced by coils of wire passing through magnetic fields inside the generator. Most alternating current generating sets are also referred to as generators.
Gigawatt (GW): A measurement of power equal to a thousand million Watts.
Gigawatt-hour (GWh): A measurement of energy. One Gigawatt-hour is equal to one Gigawatt being used for a period of one hour, or one Megawatt being used for 1000 hours.
Grid: An electrical utility distribution network.
Grid-connected: An energy producing system connected to the utility transmission grid. Also called Grid tied.
Ground loop: An undesirable feedback condition caused by two or more circuits sharing a common electrical line, usually a grounded conductor.

H

Halogen lamp: A special type of incandescent globe made of quartz glass and a tungsten filament, enabling it to run at a much higher temperature than a conventional incandescent globe. Efficiency is better than a normal incandescent, but not as good as a fluorescent light.
Harmonic content: Frequencies in the output waveform in addition to the primary frequency (usually 50 or 60 Hz.) Energy in these harmonics is lost and can cause undue heating of the load.
Head: The vertical distance that water will fall from the inlet of the collection pipe to the water turbine in a hydro power system.
Hertz (Hz): Unit of measurement for frequency. Home mains power is normally 50Hz in Europe and 60Hz in the USA. The magnitude of the current is measured in Amps.
High voltage disconnect: Voltage at which the charge controller will disconnect the array to prevent overcharging the batteries.
Hot spot: A phenomenon where one or more cells within a PV module or array act as a resistive load, resulting in local overheating or melting of the cells.
Hybrid system: A PV system that includes solar PV and some other electricity generating power source.

I

Incandescent light: an electric lamp which is evacuated or filled with an inert gas and contains a filament (commonly tungsten). The filament emits visible light when heated to extreme temperatures by passage of electric current through it.
Incident light: Light that shines on to the surface of a PV cell or module.
Independent power system: A power generation system that is independent of the mains grid.
Insolation: The amount of sunlight reaching an area, usually expressed in Watt hours per square meter per day.
Installer: Usually a retailer and installer of PV Systems
Insulation: A material used to prevent the flow of electricity. Normally used on electrical wires to prevent electric shock. Typical materials used include plastics such as PVC and polypropylene.
Integrator: Integrates PV components in to a complete PV System
Interconnect: A conductor within a module or other means of connection which provides an electrical interconnection between the solar cells. [UL 1703]
Inverter: An inverter converts DC power from the PV array/battery to AC power. Used either for stand-alone systems or grid-connected systems.
Irradiance: The solar power incident on a surface, usually expressed in kilowatts per square meter. Irradiance multiplied by time gives insolation.
I-V curve: A graph that plots the current versus the voltage from a PV cell as the electrical load (or resistance) is increased from short circuit (no load) to open circuit (maximum voltage). The shape of the curve characterizing cell performance. Three important points on the I-V curve are the open-circuit voltage, short-circuit current, and peak or maximum power (operating) point.
I-V data: The relationship between current and voltage of a photovoltaic device in the power-producing quadrant, as a set of ordered pairs of current and voltage readings in a table, or as a curve plotted in a suitable coordinate system [ASTM E 1036]

J

Joule (J): The energy conveyed by one Watt of power for one second, unit of energy equal to 1/3600 kilowatt-hours.
Junction box: A PV junction box is a protective enclosure on a PV module where PV strings are electrically connected and where electrical protection devices such as diodes can be fitted.
Junction diode: A semiconductor device, having a junction and a built-in potential, that passes current better in one direction than the other. All solar cells are junction diodes.

K

Kilowatt(kW): A unit of electrical power, one thousand Watts.  
Kilowatt-hour (kWh): The amount of energy that derives from a power of one thousand Watts acting over a period of 1 hour. The kWh is a unit of energy. 1 kWh=3600 kJ.

L

Langley: Unit of solar irradiance, one calorie per square centimeter. 1 L = 41.84 kJ/m2.
Lead-acid battery: A type of battery that consists of plates made of lead, lead-antimony, or lead-calcium and lead-oxide, surrounded by a sulfuric acid electrolyte. The most common type of battery used in RAPS systems.
Life: The period during which a system can operate above a specified performance level.
Life-cycle cost: The estimated cost of owning, operating and disposing of a system over its useful life.
Light emitting diode: A semi conductor device composed of a p-n junction designed such that electrons emit visible light during their migration across the junction.
Light trapping: The trapping of light inside a semiconductor material by refracting and reflecting the light at critical angles; trapped light will travel further in the material, greatly increasing the probability of absorption and hence of producing charge carriers.
Light-induced defects: Defects, such as dangling bonds, induced in an amorphous silicon semiconductor upon initial exposure to light.
Line wire loss: refers to the voltage or power lost due to the resistance of any wire or wires in any electrical circuit.
Linear current booster: an electronic circuit that matches PV output directly to a motor. Used in array direct water pumping.
Liquid electrolyte battery: A battery containing a liquid solution of an electrolyte in a solvent (e.g. sulfuric acid in water). Also called a flooded battery because the plates are covered with the electrolyte solution.
Load: The electrical power being consumed at any given moment or averaged over a specified period. The load that an electric generating system supplies varies greatly with time of day and to some extent season of year. Also, in an electrical circuit, the load is any device or appliance that is using power.
Load circuit: The wiring including switches and fuses that connects the load to the power source.
Load current: The current required to power the electrical device.
Load resistance: The electrical resistance of the load. 
Low voltage cut-off: The voltage at which a controller will disconnect the load from the battery.
Low voltage disconnect (LVD): The voltage at which the charge controller will disconnect the load from the batteries to prevent over-discharging.
Low voltage warning: A warning buzzer or light that indicates the low battery voltage set-point has been reached.

M

Maintenance free battery: A sealed battery to which water cannot be added to maintain the level of the  electrolyte solution.
Maximum power point (MPP): Operating a PV array at that voltage will produce maximum power. The point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum. [UL 1703] This corresponds to the point on an I-V curve that represents the largest area rectangle that can be drawn under the curve. For a typical silicon cell panel, this is about 17 volts for a 36 cell configuration.
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 measurement of power equal to one million Watts.
Megawatt-hour (MWh): A measurement of power with respect to time (i.e. energy). One megawatt-hour is equal to one megawatt being used for a period of one hour, or one kilowatt being used for 1000 hours.
Microgroove: A small groove scribed into the surface of a cell which may be filled with metal for contacts.
Modified sine wave: A waveform with at least three states (positive, off, and negative) used to simulate a sine wave. It has less harmonic content than a square wave. This type of waveform is better than a square wave, but not as suitable for some appliances as a sine wave.
Modularity: The use of complete sub-assemblies to produce a larger system. Also the use of multiple inverters connected in parallel to service different loads.
Module: An encapsulated panel containing a number of electrically connected PV cells.
Module de-rating factor: A factor that lowers the power output of a module to account for field operating conditions e.g. dirt build-up on the module.
Monocrystalline solar cell: A form of solar cell made from a thin slice of a single large crystal of silicon.
Monolithic: Fabricated as a single structure, as used to describe thin film series interconnected PV cells on a single sheet substrate.
Multicrystalline: A material that has solidified at a rate such that many small crystals (crystallites) form. The atoms within a single crystallite are symmetrically arranged with a particular orientation, whereas the crystallites themselves are differently oriented.  The multitude of grain boundaries in the material (between the crystallites) reduce the cell efficiency. Multicrystalline is also referred to as polycrystalline.
Multi-junction device: A PV device containing two or more cell junctions each of which may be different in nature and optimized to absorb a particular part of the solar spectrum in order to achieve higher overall cell efficiency.
Multi-stage controller: A charge controller that allows different charging currents as the battery approaches full state of charge.

N

NEC: US National Electrical Code which contains guidelines for all types of electrical installations which should be followed when installing a PV system.
NEMA: US National Electrical Manufacturers Association, sets standards for some non-electronic products e.g. junction boxes.
Net metering: The practice of exporting surplus solar power during the day (to actual power needs) to the electricity grid, which either causes the home owner electric meter to (physically) go backwards and/or simply creates a financial credit on the home owner's electricity bill. (At night, the homeowner draws from the electricity grid in the normal way).
Nickel-cadmium battery (NiCad): A form of rechargable battery, having higher storage densities than that of lead-acid batteries, that uses a mixture of nickel hydroxide and nickel oxide for the anode, and cadmium metal for the cathode. The electrolyte is potassium hydroxide.
Noise: Unwanted electrical signals produced by electric motors and other machines that can cause circuits and appliances to malfunction.
Nominal voltage: A rounded voltage value used to describe batteries, modules, or systems based on their specification (e.g. a 12V, 24V or 48V battery, module, or system).
Normal Operating Cell Temperature (NOCT): The estimated temperature of a solar PV module when it is  operating under 800 W/m2 irradiance, 20°C ambient temperature and a wind speed of 1 meter per second. NOCT is used to estimate the nominal operating temperature of a module in the field.
N-type semiconductor: A semiconductor produced by the doping of an intrinsic semiconductor with an electron-donor impurity, for example phosphorous in silicon.
N-Type silicon: Silicon doped with an element that has more electrons in its atomic structure than does silicon (e.g. phosphorus).

O

Ohm: The resistance between two points of a conductor when a constant potential difference of one Volt applied between these points produces in the conductor a current of one Amp.
Ohm's Law: A simple mathematical formula that allows either voltage, current or resistance to be calculated when the other two values are known. The formula is: V = I x R, where V is the voltage, I is the current, and R is the resistance.
One-axis tracking: A PV System structure that is capable of rotating on a single axis in order to track the movement of the sun.
Open circuit voltage: The maximum voltage produced by an illuminated solar PV cell, module, or array when  no load is connected. OCV increases as the temperature of the PV material decreases.
Operating point: Defined by the current and voltage that a module or array produces when connected to a load.  It is dependent on the load or the batteries connected to the output terminals.
Orientation: Position with respect to the cardinal directions, N, S, E, W.
Overcharge: Applying current to a fully charged battery.  This can damage the battery.

P

Panel: Used interchangeably with "module".
Parallel connected: A method of connection in which positive terminals are connected together and negative terminals are connected together. Current output adds and voltage remains the same.
Passive solar home: A house that utilizes part of the building as a solar collector, as opposed to active solar, such as PV.
Peak load: The maximum usage of electrical power occurring in a given period of time, typically a day.  The electrical supply must be able to be meet the peak load if it is to be reliable.
Peak power current: Current in Amperes produced by a module or array operating at the voltage on the I-V curve that will produce its maximum power.
Peak sun hours: The equivalent number of hours per day when solar irradiance averages 1000 W/m2.
Peak Watt: The amount of power a solar PV cell or module will produce under standard test conditions (normally 1000 W/m2 and 25°C cell temperature, AM 1.5 spectrum).
Photon: Light is composed of energy particles called photons which have variable energy but constant speed.
Photovoltaic (PV) array: A number of PV modules connected together in a single structure.
Photovoltaic (PV) cell: The smallest discrete element in a PV module that performs the conversion of light into electrical energy to produce a DC current and voltage.
Photovoltaic (PV) conversion efficiency: The ratio of the electrical power generated by a PV device to the power of the light incident on it. This is typically in the range 5% to 15% for commercially available modules.
Photovoltaic (PV) generator: The total of all PV strings of a PV power supply system, which are electrically interconnected.
Photovoltaic (PV) module: A single assembly of solar cells and ancillary parts, such as interconnections, terminals, (and protective devices such as diodes) intended to generate DC power under un-concentrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate). [UL 1703]
Photovoltaic (PV) panel: a term often used interchangeably with PV module (especially in single module systems).
Photovoltaic (PV): refers to any device which produces free electrons when exposed to light.
Photovoltaic system: All the parts connected together that are required to produce solar electricity.
Photovoltaic cell: The semiconductor device that converts solar irradiance (light) into dc electricity.
Photovoltaic effect: The effect that causes a voltage to be developed across the junction of two different materials when they are exposed to light.
Photovoltaic-thermal (PV/T) system: A photovoltaic system that, in addition to converting sunlight into electricity, collects the residual heat energy and delivers both heat and electricity in usable form. Also called a total energy system.
Plates: The electrodes in a battery, usually take the form of metal plates.
Polycrystalline cell: a wafer of silicon with a multi-grained structure. All grains have the same atomic crystal lattice, however, each grain has a unique orientation in space thereby producing a unique reflection of light.
Polycrystalline silicon: A material used to make solar PV cells which consists of many crystals, compared to single crystal silicon.
Poly-vinyl chloride (PVC): A plastic used as an insulator on electrical cables. A toxic material, which is being replaced with alternatives made from more benign chemicals
Power (Watts): Basic unit of electricity equal to the product of current and voltage (in DC circuits).
Power conditioning equipment: Electrical equipment, or power electronics, used to convert power from a photovoltaic array into a form suitable for subsequent use. A collective term for inverter, converter, battery charge regulator, and blocking diode.
Power conversion efficiency: The ratio of output power to input power e.g. of an inverter.
Power density: The ratio of the power available from a battery to its volume (Watts per liter) or weight (Watts per kilogram).
Power factor: The cosine of the phase angle between the voltage and the current waveforms in an AC circuit. A measure of inverter performance. 
Power: The rate of doing work. Expressed as Watts (W). For example, a generator rated at 800 watts can provide that amount of power continuously. 1 Watt = 1 joule/sec.
Primary battery: A battery that cannot be re-charged.
PV array: two or more photovoltaic panels wired in series and or parallel.
PV: Short hand for Photovoltaics.
PV components: The individual parts of a PV System. Individual items like Batteries, Inverters, Regulators, Wiring
PV system: All the parts in combination required to generate solar electricity
Pyranometer: An instrument for measuring total hemispherical solar irradiance on a flat surface, or "global" irradiance; thermopile sensors have been generally identified as pyranometers, however, silicon sensors are also referred to as pyranometers.

Q

Qualification test (PV): A testing procedure for PV modules relating to electrical, mechanical, or thermal stress.
Quasi sine-wave: A description of the type of waveform produced by some Inverters.

R

RAPS (Remote Area Power Supply): A power generation system used to provide electricity to remote and rural homes, usually incorporating power generated from renewable sources such as solar panels and wind generators, as well as non-renewable sources such as petrol-powered generators.
Rated battery capacity (Ah): Term used by battery manufacturers to indicate the maximum amount of energy that can be withdrawn from a battery at a specified discharge rate and temperature.
Rated module current (A): The current output of a PV module measured under standard test conditions of 1000 W/m2 and 25°C cell temperature.
Rated power: Nominal power output of an inverter, some units cannot produce rated power continuously.
Reactive power: The sine of the phase angle between the current and voltage waveforms in an AC system.
Rechargeable battery: A type of battery that uses a reversible chemical reaction to produce electricity, allowing it to be re-used many times. The chemical reaction is reversed by forcing electricity through the battery in the opposite direction to normal discharge.
Rectifier: A device that converts ac to dc, as in a battery charger or converter.
Regulator: A device used to limit the current and voltage in a circuit, normally to allow the correct charging of batteries from power sources such as solar panels and wind generators.
Remote site: A site with no electrical utility grid connection.
Renewable energy: Energy that is produced from a renewable source.
Resistance (R): The property of a material which resists the flow of electric current when a potential difference is applied across it, measured in Ohms.
Resistive voltage drop: The voltage developed across a cell by the current flow through the resistance of the cell which may result from the bulk resistance of the materials in the cell and at interfaces between them.
Resistor: An electronic component used to restrict the flow of current in a circuit. Sometimes used specifically to produce heat, such as in a water heater element.
Reverse bias: Condition where the current producing capability of a PV cell is significantly less than that of other cells in its series string. This can occur when a cell is shaded, cracked, or otherwise degraded or when it is electrically poorly matched with other cells in its string.
Reverse current protection: Any means of preventing current flow from the battery to the solar PV array (e.g. at night) that would discharge the battery.

S

Sacrificial anode: A piece of metal electrically connected to a buried or submerged structure that is to be protected from corrosion. The metal of the sacrificial anode is selected to corrode preferentially to the metal of the protected structure and so reduce its corrosion rate.
Sealed battery: A battery with a captive electrolyte and a re-sealing vent cap to which electrolyte cannot be added. Also called a valve-regulated battery. 
Sealed lead-acid battery: A form of lead-acid battery where the electrolyte is immobilized, either by being contained in an absorbent fibre separator or gel between the batteries plates.
Secondary battery: A battery that can be recharged; a rechargeable battery.
Self discharge: Self discharge represents energy lost to internal chemical reactions within the cell.
Self discharge rate: The rate at which a battery will lose its charge when at open circuit (with no load connected).
Semiconductor: A material that has an electrical conductivity in between that of a metal and an insulator. Transistors and other electronic devices are made from semiconducting materials, and are often called semiconductors. Typical semiconductors for PV cells include silicon, gallium arsenide, copper indium diselenide, and cadmium telluride.
Series connected: A method of connection in which the positive terminal of one device is connected to the negative terminal of another. The voltages add and the current is limited to the least of any device in the string.
Series regulator: A type of battery charge controller or regulator in which the charging current is controlled by a switch, transistor, or field-effect transistor connected in series with the PV module or array.
Series resistance: Resistance to current flow within a cell due to factors such as the bulk resistance of the cell materials and contact resistances.
Shallow-cycle battery: A battery with small plates that cannot withstand many deep discharges (i.e. to a low state of charge).
Shelf life: The time for which a device can be stored and still retain its specified performance.
Short circuit current (Isc) : The current generated by an illuminated solar PV cell, module, or array when its output terminals are shorted; the maximum current possible.
Shunt controller: A controller or regulator that re-directs, or shunts, the charging current away from the battery. Generally used for smaller systems.
Silicon (Si): A chemical element with atomic number 14, a dark gray semi-metal. Occurs in a wide range of silicate minerals and makes up approximately 28% of the earth's crust (by weight).  Silicon has a face-centered cubic lattice structure like diamond. The most common semiconductor material used in making PV cells either traditionally in its crystalline form or more recently as an amorphous thin film.
Sine wave: A waveform that has is defined by an equation in which one variable is proportional to the sine of the other, as generated by an oscillator in simple harmonic motion. The sine wave is the most ideal form of electricity for running more sensitive appliances, such as radios, TVs, computers and the like.
Sine wave inverter: An inverter that produces grid-quality, sine wave AC electricity.
Single-crystal silicon: Silicon material with a single crystal structure. A common material for the construction of solar PV cells.
Solar constant: The power density of solar radiation on a plane perpendicular to the direction of the sun at the mean earth-sun distance outside the earth's atmosphere; its value is 1.37 kW per square meter.
Solar energy: Energy from the sun.
Solar noon: The mid-point between sunrise and sunset; the time when the sun reaches its highest point in its daily arc across the sky.
Solar power: Electricity generated by conversion of sunlight, either directly through the use of photovoltaic panels, or indirectly through solar-thermal processes.
Solar module: A device used to convert light from the sun directly into DC electricity by using the photovoltaic effect. Usually made of multiple solar cells bonded between glass and a backing material. A typical Solar Module would be 100 Watts of power output (but module powers can range from 1 Watt to 300 Watts) and have dimensions of 2 feet by 4 feet.
Solar resource: The amount of solar insolation received at a site, normally measured in units of kWh/m2/day which equates to the number of peak sun hours.
Solar spectrum: The total distribution of electromagnetic radiation emanating from the sun.
Solar thermal electric: Method of producing electricity from solar energy by using focused sunlight to heat a working fluid, which in turn drives a turbogenerator.
Solar thermal: A form of power generation using concentrated sunlight to heat water or other fluid that may then used to drive a motor or turbine.
Solar-grade silicon: Intermediate-grade silicon used in the manufacture of solar cells. Less expensive than electronic-grade silicon.
Specific gravity: The ratio of the weight of a solution to the weight of an equal volume of water at a specified temperature; used with reference to the sulfuric acid electrolyte solution in a lead acid battery as an indicator of battery state of charge.  More recently called relative density.
Split-spectrum cell: A compound photovoltaic device in which sunlight is first divided into spectral regions by optical means. Each region is then directed to a different photovoltaic cell optimized for converting that portion of the spectrum into electricity. Such a device achieves significantly greater overall conversion of incident sunlight into electricity.
Square wave: A train of rectangular voltage pulses that alternate between two fixed values for equal lengths of time.
Square wave inverter: The simplest and the least expensive type of inverter, but which produces the lowest quality of power.  The inverter uses switches that can carry a large current and withstand a high voltage that are turned on and off in the correct sequence and at a certain frequency. 
Stand-alone (PV system): A solar PV system that operates without connection to a grid a supply of electricity.
Standard test conditions (STC): Conditions under which a module is typically tested in a laboratory: (1) Irradiance intensity of 1000 W/square meter (0.645 watts per square inch), AM1.5 solar reference spectrum, and (3) a cell (module) temperature of 25 degrees C, plus or minus 2 degrees C (77 degrees F, plus or minus 3.6 degrees F). [IEC 1215]
Standby current: The current used by the inverter when no load is active, corresponding to lost power. 
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.
State of charge (SOC): The capacity of a battery at a particular time expressed at a percentage of its rated capacity.
Static head: The height of the water level above the point of free discharge of the water, normally measured when the pump is off.
Storage: Storing energy in a battery or battery stack. In water pumping, storage can be achieved by pumping water to a storage tank.
Storage density: The capacity of a battery, in amp-hours compared to its weight. Measured in Watt-hours per kilogram.
Stratification: Occurs in a liquid electrolyte solution when its concentration varies from top to bottom. Can be solved by periodic controlled charging at voltages that produce gassing to mix the electrolyte solution.
String: A number of cells, modules or panels interconnected electrically in series to produce the required operating voltage.
Substrate: The physical material upon which a photovoltaic cell is made. Sub-system: Any one of several components in a PV system (i.e., array, controller, batteries, inverter, load).
Suction head: The height of pump above the surface of the water source when the pump is located above the water level.
Sulfation: The formation of lead-sulfate crystals on the plates of a lead-acid battery; large crystals of lead sulfate grow on the plate, instead of the usual tiny crystals, making the battery extremely difficult to recharge. If the crystals get large enough, shorting of the cell may occur.
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.
Surge capacity: The ability of an inverter or generator to deliver instantaneous high currents when starting motors, for example.
Surge: An excessive amount of power drawn by an appliance when it is first switched on. An unexpected flow of excessive current, usually caused by excessive voltage, that can damage appliances and other electrical equipment.
Switch: a common device which breaks an electrical circuit thereby halting the flow electricity through the circuit.
Switch-mode: A form of converting one form of electricity to another by rapidly switching it on and off and feeding it through a transformer to effect a voltage change.
System availability: The proportion of time (usually expressed in hours per year) that a solar PV system will be able to meet fully the load demand.
System operating voltage: The output voltage of a solar PV array under load, dependent on the electrical load and size of the battery stack connected to the output terminals.

T

Temperature compensation: Adjustment via the use of electronic circuitry to change the charge controller activation points depending on battery temperature. This is desirable if the battery temperature is expected to vary by more than 5 deg C from the ambient temperature. The temperature coefficient for lead acid batteries is typically -3 to -5 millivolts/deg C per cell.
Temperature factors: Are used to decrease battery capacity at cold temperatures, to decrease PV module voltage at high temperatures and to increase the resistance of wire at high temperatures.
Thermal electric: Electric energy derived from heat energy, usually by heating a working fluid, which drives a turbogenerator.
Thermophotovoltaic (TPV) device: A device in which solar energy is concentrated on to a radiator which reaches a high temperature and emits the energy in a different part of the spectrum, better matched to the bandgap of the matched solar cell.  This approach should enable high cell efficiencies to be obtained.
Thick cells : Conventional solar cells in most types of PV modules, such as crystalline silicon cells, which are typically from 200-400 micrometers thick. In contrast, thin-film cells are several microns thick.
Thick-crystalline materials: Semiconductor material, typically measuring from 200-400 micromterers thick, that is cut from boules, ingots or ribbons.
Thin film PV module: A solar PV module constructed with sequential layers of thin film semiconductor materials usually only micrometers thick. Currently, thin film technologies account for around 12% of all solar modules sold around the world. This share is expected to increase, since thin film technologies represent a potential route to lower costs.
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.
Tilt angle: The angle of inclination of a solar collector or solar module measured from the horizontal.
Total AC load demand: The sum of the AC loads; its value is important to select the correct Inverter.
Total internal reflection: The trapping of light within the PV cell by internal reflection of incident light at angles greater than the critical angles for the interfaces, so that the light cannot escape the cell and is therefore eventually absorbed by the semiconductor.
Tracker: any device used to direct a PV array towards the sun.
Tracking array: A PV array that is moved to follow the path of the sun in order to maintain the maximum incident solar radiation on its surface. The two most common methods are firstly single-axis tracking in which the array tracks the sun from east to west, and secondly, two-axis tracking in which the array points directly at the sun all the time. Two-axis tracking arrays capture the maximum possible daily energy. Typically, a single axis tracker will give 15% to 25% more power per day, and dual axis tracking will add a further 5%.
Transformer: A transformer is a device that changes voltage from one level to another. A device used to transform voltage levels to facilitate the transfer of power from the generating plant to the customer.
Transistor: A semi-conductor device used to switch or otherwise control the flow of electricity.
Trickle charge: A small charging current designed to keep a battery fully charged.
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.

U

Uninterruptible power supply (UPS): A power supply capable of providing continuous uninterruptible service; normally containing batteries to provide energy storage.
Utility-interactive inverter: An inverter that can operate only when connected to the utility grid supply and an output voltage frequency fully synchronized with the utility power.

V

VAC: Volts alternating current
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.
VDC: Volts direct current
Vented cell: A battery with a vent to expel gases liberated during charging.
Vmp: The voltage at which a PV device is operating at maximum power.
Voc: Open-circuit voltage Volt (V): The unit of electromotive force that will force a current of one ampere through a resistance of one ohm. Voltage at maximum power
Volt: The unit if electric potential and potential difference. The amount of work done per unit charge in moving a charge from one place to another.  The potential difference across a resistance of 1 Ohm when a current of 1 Amp is flowing.
Voltage drop: The voltage lost along a length of wire or conductor due to the resistance of that conductor. This also applies to resistors. The voltage drop is calculated by using Ohm's Law.
Voltage protection: A sensing circuit on an Inverter that will disconnect the unit from the battery if input voltage limits are exceeded.
Voltage regulator: A device that controls the operating voltage of a photovoltaic array.
Voltage: Unit of measurement for the electrical `pressure' of electricity. Measured in Volts (V).
Voltmeter: An electrical or electronic device used to measure voltage.

W

Wafer: A thin sheet of crystalline semiconductor material either made by mechanically sawing it from a single-crystal boule or multicrystalline ingot or block, or made directly by casting. The wafer is "raw material" for the solar cell.
Watt (W) : The unit of electrical power commonly used to define the electricity consumption of an appliance. The power developed when a current of one ampere flows through a potential difference of one volt; 1/746 of a horsepower. 1 Watt = 1 Joule/s.
Watt hour (Wh): A unit of energy equal to one Watt of power being used for one hour. 
Waveform: The shape of a wave or pattern representing a vibration. The shape characterizing an AC current or voltage output.
Watt peak (User friendly definition): Is the Direct Current Watts output of a Solar Module as measured under an Industry standardized Light Test before the Solar Module leaves the Manufacturers facility.
Watt peak: (technical definition): The Watt Power output of a Solar module is the number of Watts Output when it is illuminated under standard conditions of 1000 Watts/meter2 intensity, 25°C ambient temperature and a spectrum that relates to sunlight that has passed through the atmosphere (AM or Air Mass 1.5).
Wet shelf life: The period over which a charged battery, filled with electrolyte, can remain unused before its performance falls below a specified.

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