Solar Electricity Glossary
-A-
Absorbed glass mat (AGM): A fibrous silica glass mat to suspend the electrolyte in batteries. This mat provides pockets that assist in the recombination of gasses generated during charging back into water.
Air mass: (Sometimes called air mass ratio) — Equal to the cosine of the zenith angle-that angle from directly overhead to a line intersecting the sun. The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness).
Alternating current (AC): Electric current in which the direction of flow is reversed at frequent intervals, usually 100 or 120 times per second (50 or 60 cycles per second or 50/60 Hz).
Altitude: The angle between the horizon (a horizontal plane) and the sun’s position in the sky. Measured in degrees.
Amorphous silicon: A non-crystalline semiconductor material that is of ten used in thin film photovoltaic modules.
Ampere (A) or amp: 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. Sometimes this is abbreviated as I for intensity.
Ampere-hour (Ah): Quantity of electrical energy equal to the flow of one ampere of current for one hour. Typically used to quantify battery bank capacity.
Angle of incidence: Angle which references the sun’s radiation striking a surface. A “normal” angle of incidence refers to the sun striking a surface at a 90° angle.
Array: Any number of photovoltaic modules connected together to provide a single electrical output at a specified voltage. Arrays are of ten designed to produce significant amounts of electricity.
Avoided cost: The minimum amount an electric utility is required to pay an independent power producer, under the PURPA regulations of 1978, equal to the costs the utility calculates it avoids in not having to produce that power (usually substantially less than the retail price charged by the utility for power it sells to customers).
Azimuth: Angle between true south and the point directly below the location of the sun. Measured in degrees east or west of true south in northern latitudes.
-B-
Balance of system (BOS): All system components and costs other than the PV modules.
Battery: Two or more “cells” electrically connected for storing electrical energy. Common usage permits this designation to be applied also to a single cell used independently, as in a flashlight battery.
Battery capacity: The total number of ampere-hours that can be withdrawn from a fully charged cell or battery.
Battery cell: A galvanic cell for storage of electrical energy. This cell, after being discharged, may be restored to a fully charged condition by an electric current.
Battery cycle life: The number of cycles, to a specified depth of discharge, that a cell or battery can undergo before failing to meet its specified capacity or efficiency performance criteria.
Battery self-discharge: The rate at which a battery, without a load, will lose its charge.
Battery state of charge: Percentage of full charge or 100 percent minus the depth of discharge.
Blocking diode: A semiconductor device connected in series with a PV module and a storage battery to prevent a reverse current discharge of the battery through the module when there is no output, or low output from the cells. When connected in series to a PV string, it protects its modules from a reverse power flow preventing against the risk of thermal destruction of solar cells.
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. Alternatively, a diode connected anti -parallel across a part of the solar cells of a PV module. It protects these solar cells from thermal destruction in case of total or partial shading of individual solar cells while other cells are exposed to full light.
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Cell: The basic unit of a photovoltaic module. This word is also commonly used to describe the basic unit of batteries (ie. a 6 -volt battery has (3) 2-volt cells).
Charge controller: A device that controls the charging rate and/or state of charge for batteries.
Charge rate: The current applied to a cell or battery to restore its available capacity.
Concentrator: A PV module that uses optical elements to increase the amount of sunlight incident on a PV cell. Concentrating arrays must track the sun and use only the direct sunlight because the diffuse portion cannot be focused onto the PV cells.
Conversion efficiency: The ratio of the electric energy produced by a photovoltaic device (under full sun conditions) to the energy from sunlight incident upon the cell.
Crystalline silicon: A type of PV cell made from a single crystal or polycrystalline slice of silicon.
Current: The flow of electric charge in a conductor between two points having a difference in potential (voltage).
Current at maximum power (Imp): The current at which maximum power is available from a module. [UL 1703]
Cycle life: Number of discharge-charge cycles that a battery can tolerate under specified conditions before it fails to meet specified criteria as to performance (e.g., capacity decreases to 80% of the nominal capacity).
-D-
Days of autonomy: The number of consecutive days a stand-alone system battery bank will meet a defined load without solar energy input.
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).
Deep cycle battery: Type of battery that can be discharged to a large fraction of capacity many times without damaging the battery.
Demand Load: The total power required by a facility.
Depth of discharge (DOD): The amount of ampere hours removed from a fully charged cell or battery. Expressed as a percentage of rated capacity.
Diode: Electronic component that allows current flow in one direction only.
Direct current (DC): Electric current in which electrons flow in one direction only. Opposite of alternating current.
Discharge rate: The rate, usually expressed in amperes over time, at which electrical current is taken from the battery.
Disconnect: Switch gear used to connect or disconnect components of a PV system for safety or service.
Dual-axis tracking: A system capable of rotating independently about two axes and following the sun’s orientation and height in the sky (e.g., vertical and horizontal).
Duty cycle: The ratio of active time to total time. Used to describe the operating regime of appliances or loads.
-E-
Efficiency: The ratio of output power to input power. Expressed as a %.
Electric current: A flow of electricity.
Electrical grid: An integrated system of electricity distribution, usually covering a large area.
Electrolyte: A liquid conductor of electricity in which flow of current takes place by migration of ions. The electrolyte for a lead-acid storage cell is an aqueous solution of sulfuric acid.
Energy: The ability to do work. Stored energy becomes working energy when we use it.
Equalization: The process of mixing the electrolyte in batteries by periodically overcharging the batteries for a short period to “refresh” cell capacity.
-F-
Float life: Number of years a battery can keep its stated capacity when it is kept at float charge.
-G-
Gassing current: Portion of charge current that goes into electrolytical production of hydrogen and oxygen from the electrolytic liquid in the battery. This current increases with increasing voltage and temperature.
Gel-type battery: Lead-acid battery in which the electrolyte is composed of a silica gel matrix.
Gigawatt (GW): One billion watts. One million kilowatts. One thousand megawatts.
Grid-connected /Grid-Interaction /Grid-Tied: A PV system in which the PV array acts like a central generating plant, supplying power to the grid.
Grid-interactive: See ‘grid-connected (PV system).’
-H-
Hybrid system: A PV system that includes other sources of electricity generation, such as wind or fossil fuel generators.
-I-
Insolation: Sunlight, direct or diffuse. From incident solar radiation: usually expressed in watts per square meter. Not to be confused with ‘insulation’.
Interconnect: A conductor within a module or other means of connection which provides an electrical interconnection between the solar cells.
Inverters: Devices that convert DC electricity into AC electricity (single or multiphase). Either for stand-alone systems (not connected to the grid) or for utility-interactive systems.
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. Typically measured at 1,000 watts per square meter of solar insolation at a specific cell temperature. The shape of the curve characterizes cell performance.
-J-
Junction box: An electrical box designed to be a safe enclosure in which to make proper electrical connections. On PV modules, this is where PV strings are electrically connected.
-K-
Kilowatt (kW): 1,000 watts.
Kilowatt-hour (kWh): One thousand watt hours. The kWh is a unit of energy. 1 kWh = 3,600 kJ.
-L-
Life cycle cost: An estimate of the cost of owning and operating a system for the period of its useful life. Usually expressed in terms of the present value of all lifetime costs.
Load: Anything in an electrical circuit that, when the circuit is turned on, draws power from that circuit.
-M-
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. For a typical silicon cell, this is at about 0.45 V.
Maximum power point tracker (MPPT): Means of a power conditioning unit that automatically operates the PV generator at its MPP under all conditions.
Megawatt (MW): One million watts. One thousand kilowatts.
Module: See ‘photovoltaic module’.
Multi-crystalline: Material that is solidified at such a rate that many small crystals (crystallites) form. The atoms within single crystallites 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 poly-crystalline or semi -crystalline).
-N-
NEC: An abbreviation for the National Electrical Code which contains safety guidelines and required practices for all types of electrical installations.
Nominal operating cell temperature (NOCT): The reference cell (module) operating temperature presented on manufacturers literature. Generally, the NOCT is referenced at 25°C, 77°F.
Nominal voltage: A reference voltage used to describe batteries, modules, or systems (ie. a 12-, 24-,or 48-volt battery, module or system).
-O-
Ohm: The unit of resistance to the flow of an electric current.
Open-circuit voltage (Voc): The maximum possible voltage across a photovoltaic cell or module; the voltage across the cell in sunlight when no current is flowing.
Orientation: Placement according to the compass directions - north, south, east, west.
-P-
Parallel connection: A way of joining two or more electricity-producing devices such as PV cells or modules, or batteries by connecting positive leads together and negative leads together; such a configuration increases the current but the voltage is constant.
Peak load; peak demand: The maximum load, or usage, of electrical power occurring in a given period of time, typically a day.
Peak sun hours: The equivalent number of hours per day when solar irradiance averages 1000 w/m (full sun).
Photovoltaic (PV): Pertaining to the direct conversion of sunlight photons into electricity.
Photovoltaic array: An interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure, with common support or mounting. In smaller systems, an array can consist of a single module.
Photovoltaic cell: The smallest semiconductor element within a PV module to per form the immediate conversion of light into electrical energy (DC voltage and current).
Photovoltaic module: The smallest environmentally protected, essentially planar assembly of solar cells and ancillary parts, such as interconnections, terminals, and protective devices such as diodes intended to generate DC power under unconcentrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate).
Photovoltaic peak watt: Maximum rated output of a cell, module, or system. Typical rating conditions are 0.645 watts per square inch (1000 watts per square meter) of sunlight, 68 degrees F (20 degrees C) ambient air temperature and 6.2 x 10-3 mi/s (1 m/s) wind speed.
Photovoltaic system: A complete set of components for converting sunlight into electricity by the photovoltaic process, including the array and balance of system components.
Poly-crystalline: See ‘multi-crystalline.’
Power factor: The ratio of the average power and the apparent volt-amperes.
Pulse-width-modulated wave inverter (PWM): PWM inverters are the most expensive, but produce a high quality output signal at minimum current harmonics. The output voltage is very close to sinusoidal.
PV: Abbreviation for photovoltaic.
-R-
Resistance (R): The property of a conductor which opposes the flow of an electric current resulting in the generation of heat in the conducting material. The unit of resistance is ohms.
-S-
Semiconductor: Any material that has a limited capacity for conducting an electric current. Certain semiconductors, including silicon, gallium arsenide, copper indium dislenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process.
Series connection: A way of joining electrical equipment by connecting positive leads to negative leads; such a configuration increases the voltage while current remains the same.
Series regulator: Type of battery charge regulator where the charging current is controlled by a switch connected in series with the PV module or array.
Shelf life of batteries: The length of time, under specified conditions, a battery can be stored so it keeps its guaranteed capacity.
Short-circuit current (Isc): The current flowing freely from a photovoltaic cell through an external circuit that has no load or resistance; the maximum current possible.
Shunt regulator: Type of a battery charge regulator where the charging current is controlled by a switch connected in parallel with the PV generator. Overcharging of the battery is prevented by shorting the PV generator.
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.
Sine wave inverter: An inverter that produces utility-quality sine wave power forms.
Single-axis tracking: A system capable of rotating about one axis, also referred to as one axis. These tracking systems usually follow the sun from east to west throughout the day.
Single-crystal material: A material is composed of a single crystal or a few large crystals.
Solar cell: See ‘photovoltaic cell.’
Solar constant: The strength of sunlight; 1353 watts per square meter in space and about 1000 watts per square meter at sea level at the equator at solar noon.
Solar noon: That moment of the day that divides the daylight hours for that day exactly in half. To determine solar noon, calculate the length of the day from the time of sunset and sunrise and divide by two. The moment the sun is highest in the sky.
Square wave inverter: The square wave inverter is the simplest and the least expensive to purchase, but it produces the lowest quality of power. This inverter consists of a DC source, four switches, and the load. The switches are power semiconductors that can carry a large current and withstand a high voltage rating. The switches are turned on and off at a correct sequence, at a certain frequency.
Stand-alone: An autonomous or hybrid photovoltaic system not connected to a grid. Some stand-alone systems require batteries or some other form of storage. Also called, “stand-alone PV system.”
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°C, plus or minus 2°C (77°F, plus or minus 3.6°F).
State of charge (SOC): The available capacity remaining in a cell or battery, expressed as a percentage of the rated capacity. For example, if 25 amp-hours have been removed from a fully charged 100 amp-hour cell, the state of charge is 75 percent.
Sulfation: A condition that afflicts unused and discharged batteries; large crystals of lead sulfate grow on the plate, instead of the usual tiny crystals, making the battery extremely difficult to recharge.
Surge: The momentary start-up condition of a motor requiring a large amount of electrical current.
Surge capacity: The ability of an inverter or generator to deliver high currents momentarily required when star ting a motor.
-T-
Temperature compensation: An allowance made in charge controller set points for changing battery temperatures.
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: Angle of inclination of collector as measured in degrees from the horizontal. For maximum performance solar collectors/modules should be set perpendicular to the sun.
Total harmonic distortion (thd): The measure of closeness in shape between a waveform and its fundamental component.
Tracking PV 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 (I) 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 time. Tracking arrays use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy.
Transformer: An electromagnetic device used to convert AC electricity, either to increase or decrease the voltage.
Trickle charge: A charge at a low rate, balancing through self- discharge losses, to maintain a cell or battery in a fully charged condition.
-U-
Uninterruptible power supply (UPS): The designation of a power supply providing continuous uninterruptible service when a main power source is lost.
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.
-V-
VAC: Volts AC.
VDC: Volts DC.
VOC: Open-circuit voltage.
Volt (V): A unit of measure of the force, or ‘push,’ given the electrons in an electric circuit. One volt produces one ampere of current when acting against a resistance of one ohm.
Voltage at maximum power (Vmp): The voltage at which maximum power is available from a module.
-W-
Watt (W): The unit of electric power, or amount of work. One ampere of current flowing at a potential of one volt produces one watt of power.
Watt-hour (Wh): A quantity of electrical energy when one watt is used for one hour.
Waveform: The shape of the curve graphically representing the change in the AC signal voltage and current amplitude, with respect to time.