Solar glossary temrs

AC (Alternating Current) The direction of electrical current reverses, usually many (60) times per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.

AMPERE (amp) A unit of electrical current or rate of flow of electrons. One volt across one ohm of resistance causes a current flow of one ampere. One ampere is equal to 6.235 x 10 18 electrons per second passing a given point in a circuit.

AMPERE HOUR (amp hr. or AH), a measure of current over time, used to measure battery capacity.

AMPERE HOUR METER An instrument that monitors current with time. The indication is the product of current (in amperes) and time (in hours).

ANGLE OF INCIDENCE The angle between the direct solar beam and the normal (90 degrees) to the active surface. (degrees)

ARRAY Any number of Photovoltaic modules connected together electrically to provide a single electrical output. An array is a mechanically integrated assembly of modules or panels together with support structure (including foundation and other components, as required) to form a free-standing field installed unit that produces DC power.

AZIMUTH 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.


BALANCE OF SYSTEMS (BOS) Parts or components of a photovoltaic system other than the photovoltaic array.

BATTERY Two or more electrochemical cells enclosed in a container and electrically interconnected in an appropriate series/parallel arrangement to provide the required operating voltage and current levels. Under common usage, the term battery also applies to a single cell if it constitutes the entire electrochemical storage system.

BATTERY CAPACITY The maximum total electrical charge, expressed in ampere-hours (AH), that a battery can deliver to a load under a specific set of conditions.

BATTERY CELL The simplest operating unit in a storage battery. It consists of one or more positive electrodes or plates, an electrolyte that permits ionic conduction, one or more negative electrodes or plates, separators between plates of opposite polarity, and a container for all the above.

BATTERY AVAILABLE CAPACITY The total maximum charge, expressed in ampere-hours, that can be withdrawn from a cell or battery under a specific set of operating conditions including discharge rate, temperature, initial state of charge, age, and cutoff voltage.

BATTERY ENERGY CAPACITY The total energy available, expressed in watt-hours (kilowatt-hours), that can be withdrawn from a fully-charged cell or battery. The energy capacity of a given cell varies with temperature, rate, age, and cutoff voltage. This term is more common to system designers than it is to the battery industry where capacity usually refers to ampere-hours.

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 LIFE The period during which a cell or battery is capable of operating above a

specified capacity or efficiency performance level. For example, with lead-acid batteries, end-of-life is generally taken as the point in time when a fully charged cell can deliver only 80% of its rated capacity. Beyond this state of aging, deterioration and loss of capacity begins to accelerate rapidly. Life may be measured in cycles and/or years, depending on the type of service for which the cell or battery is intended.

BLOCKING DIODE A semiconductor connected in series with a solar cell or cells and a storage battery to keep the battery from discharging through the cell when there is no output, or low output, from the solar cell. It can be thought of as a one-way valve that allows electrons to flow forwards, but not backwards.


CATHODIC PROTECTION Systems that protect underground metal from corrosion by running small electrical currents along the metal. Most often used to protect well heads, oil, gas, and water pipelines.

CELL (battery) A single unit of an electrochemical device capable of producing direct voltage by converting chemical energy into electrical energy. A battery usually consists of several cells electrically connected together to produce higher voltages. (Sometimes the terms cell and battery are used interchangeably).

CELL (solar) The smallest, basic Photovoltaic device that generates electricity when exposed to light.

CHARGE RATE The current applied to a cell or battery to restore its available capacity. This rate is commonly normalized by a charge control device with respect to the rated capacity of the cell or battery.

CHARGE CONTROLLER A component of photovoltaic system that controls the flow of current to and from the battery to protect the batteries from over-charge and over-discharge. The charge controller may also indicate the system operational status.

CONCENTRATOR A photovoltaic module which includes optical components, such as lenses, to direct and concentrate sunlight onto a solar cell of smaller area. Most concentrator arrays must directly face or track the sun.


DC Direct Current A one way flow of electric current. Typical sources of direct currents are electric cells, rectified power units and direct current generators. This is the current flow produced by a solar system. To be used for typical 120 volt or 220 volt household appliances, it must be converted to AC (alternating current).

DEPTH OF DISCHARGE (DOD) The ampere-hours removed from a fully charged cell or battery, expressed as a percentage of rated capacity. For example, the removal of 25 ampere- hours from a fully charged 100 ampere-hours rated cell results in a 25% depth of discharge. Under certain conditions, such as discharge rates lower than that used to rate the cell, depth of discharge can exceed 100%.

DIFFUSE INSOLATION The radiant energy from the sky incident upon unit surface area during a specified time period (Same units as for direct insolation).

DIRECT INSOLATION The radiant energy from the sun (and a small area of sky surrounding it, defined by the acceptance angle of the pyrheliometer) incident upon unit surface area during a specified time period. (MJ/m2 per hour, day, week, month or year, as the case may be).


EFFICIENCY The ratio of power output of a Photovoltaic cell to the incident power from the sun or simulated sun sources under specified standard insolation conditions.

ELECTROLYTE The fluid used in batteries as the transport medium for positively and negatively charged ions.

ELECTRIC CURRENT The rate at which electricity flows through an electrical conductor, usually measured in amperes (amps).

ELECTRICITY Energy resulting from the flow of charge particles, such as electrons or ions.

ELECTRONS A negatively charged particle. The movement of electrons in an electrical conductor constitutes an electric current.

EQUALIZATION The process of restoring all cells in a battery to an equal state-of-charge. For lead-acid batteries, this is a charging process designed to bring all cells to 100% state-of- charge. Some battery types may require a complete discharge as a part of the equalization process.

EQUALIZING CHARGE A continuation of normal battery charging, at a voltage level slightly higher than the normal end-of-charge voltage, in order to provide cell equalization within a battery.


FLOAT SERVICE A battery operation in which the battery is normally connected to an external current source; for instance, a battery charger which supplies the battery load under normal conditions, while also providing enough energy input to the battery to make up for its internal quiescent losses, thus keeping the battery always up to full power and ready for service.

FULL SUN The full sun condition is the amount of power density received at the surface of the earth at noon on a clear day – about 100 mW/cm2. Lower levels of sunlight are often expressed as 0.5 sun or 0.1 sun. A figure of 0.5 sun means that the power density of the sunlight is one-half of that of a full sun.


GASSING The evolution of gas from one or more of the electrodes in the cells of a battery. Gassing commonly results from local action self discharge) or from the electrolysis of water in the electrolyte during charging.

GRID Transmission line network used to distribute electric power.

GRID LINES Metallic contacts fused to the surface of the solar cell to provide a low resistance path for electrons to flow out to the cell interconnect wires.


HERMETIC SEAL Being impervious to external influences. Typically associated with the sealing of a package so that oxygen, moisture, and other outside environments cannot enter the package.

HYBRID SYSTEM A power system consisting of two or more power generating subsystems (e.g., the combination of a wind turbine or diesel generator and a photovoltaic system.


INCIDENT LIGHT The incident light is the amount of light reaching an object.

INSOLATION The amount of sunlight reaching an area. Usually expressed in milliwatts per square centimeter, or langleys.

INVERTER A device that converts electricity from DC to AC.


junction box–A PV generator junction box is an enclosure on the module where PV strings are electrically connected and where protection devices can be located, if necessary.

junction diode–A semiconductor device with a junction and a built-in potential that passes current better in one direction than the other. All solar cells are junction diodes.


KILOWATT (kW) 1000 watts.

KILOWATT-HOUR Unit of energy used to perform work (energy and work are equivalent in units, energy being the potential value and work the achieved value). Fuel equivalents:

    • One barrel of crude contains roughly 1700 kWh
    • One ton of coal contains roughly 7500 kWh
    • One gallon of gasoline contains roughly 37 kWh
    • One cubic foot of natural gas contains 0.3 kWh
    • One ton of uranium ore contains 164 million kWh 1.34 horsepower-hours.

Thermal Unit: One kWh = 3400 BTU. Can be compared to 860 calories (food value)

Example: The “average” American home uses about 24 kWh of electricity per day. Heating 3 gallons of water from 75 degrees to the boiling point requires 1 kWh. A 200 watt Photovoltaic array of six, 50 watt Solar modules, will generate 1 kWh in an “average” day (annualized average equivalent of 5 hours peak sunlight per day).


LOAD Refers to equipment that is powered by electricity. Usually expressed in terms of amperes or watts. In an electrical circuit, any devise or appliance that uses power (such as a light bulb or water pump).


MAXIMUM POWER The power at the point on the current-voltage characteristic where the product of current and voltage is a maximum (measured in watts).

MODULE The smallest non divisible, self-contained and environmentally protected physical structure housing interconnected Photovoltaic cells and providing a single DC electrical output.

MULTICRYSTALINE 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 aspolycrystalline or semicrystalline).


OPEN CIRCUIT VOLTAGE (VOC) Voltage produced by a Photovoltaic cell with no load applied when the cell is exposed to standard insolation conditions, measured with a voltmeter.


PANEL A collection of one or more modules fastened together into a single unit, often factory pre- assembled and wired, forming a field-installable unit.

PARALLEL CONNECTION A wiring configuration used to increase current (amperage). Parallel wiring is positive to positive (+ to +) and negative to negative (- to -). Opposite of a series connection.

PEAK POWER POINT Operating point of the I-V (current-voltage) curve for a Photovoltaic cell or module where the product of the current value times the voltage value is a maximum.

PEAK WATTS The measurement of electricity produced by a solar generator at noon on a sunny day, under predetermined standard conditions.

PHOTON The actual (physical) particle unit of light, as the electron is of electric charge and the atom and molecule are of matter. Light has both wave properties and particle properties. Violet light has relatively short wavelength and higher energy in its photons; red light has longer wavelength, lower-energy photons. The wavelength and/or energy spectrum of the sun extends in both directions beyond the visible range of light, of course, and the silicon module solar cell can capture some energy in both of these invisible zones. Photons not captured by the cell are either reflected or converted to heat in the solar array.

PHOTOVOLTAIC CELL A device composed of specially prepared semiconductor material or material combinations exhibiting the ability to convert incident solar energy directly into electrical energy.

PHOTOVOLTAIC EFFECT The phenomenon that occurs when photons, the “particles” in a beam of light, knock electrons loose from the atoms they strike. When this property of light is combined with the properties of semiconductors, electrons flow in one direction across a junction, setting up a voltage. With the addition of circuitry, current will flow and electric power will be available.

PHOTOVOLTAIC SYSTEM An installed aggregate of solar arrays generating power for a given application. A system may include the following sub-systems:



    • Support Foundation
    • Power conditioning and control equipment
    • Storage
    • Active Thermal control
    • Land security systems and buildings
    • Conduit/wiring
    • Instrumentation



POWER CONDITIONER The electrical equipment used to convert power from a photovoltaic array into a form suitable for subsequent use. Loosely, a collective term for inverter, transformer, voltage regulator and other power controls.

POWER FACTOR The ratio of real power (watts) to apparent power (volt-amps) in an AC circuit. Displacement power factor is the ratio of fundamental watts to fundamental RMS volts times.


RMS Amps, excluding the effects of all harmonic exponents; it could be called fundamental power factor.

REGULATOR Prevents overcharging of batteries by controlling charge cycle-usually adjustable to conform to specific battery needs.

RENEWABLE ENERGY Flows of energy that are regenerative or virtually inexhaustible. Most commonly includes solar (electric and thermal), biomass, geothermal, wind, tidal, wave, and hydro power sources.


SEMICONDUCTOR A material such as silicon, which has a crystalline structure that will allow current to flow under certain conditions. Semiconductors are usually less conductive than metals but not an insulator like rubber.

SERIES CONNECTION A wiring configuration used to increase voltage. Series wiring is positive to negative (+ to -) or negative to positive (- to +). Opposite of parallel connection.

SILICON A non-metallic element, that when specially treated, is sensitive to light and capable of transforming light into electricity. Silicon is the basic material of most beach sand, and is the raw material used to manufacture most photovoltaic cells.

SINGLE-CRYSTAL STRUCTURE A material having a crystalline structure such that a repeatable or periodic molecular pattern exists in all three dimensions.

SOLAR CELL The basic photovoltaic device which generates electricity when exposed to sunlight.

SOLAR PANEL A collection of solar modules connected in series, in parallel, or in series- parallel combination to provide greater voltage, current, or power than can be furnished by a single solar module. Solar panels can be provided to furnish any desired voltage, current, or power. They are made up as a complete assembly. Larger collections of solar panels are usually called solar arrays.

STAND-ALONE SYSTEM (SA) A system which operates independently of the utility lines. It may draw supplementary power from the utility but is not capable of providing power to the utility.

STORAGE BATTERY A device capable of transforming energy from electric to chemical form and vice versa. The reactions are almost completely reversible. During discharge, chemical

energy is converted to electric energy and is consumed in an external circuit or apparatus.

SULFATION The formation of lead-sulfate crystals on the plates of a lead-acid battery. Commonly used to indicate the large crystals which form in partially discharged cells as the result of temperature cycling. These large crystals are more difficult to reduce by the charging current than are the smaller crystals that result from normal and self-discharge reactions. Sulfating can be caused by leaving the battery in a discharged state for long periods of time.


TELEMETRY DEVICE Devices used to transmit or receive data in a digital form.

TILT ANGLE A fixed angle measured from the horizontal to which a solar array is tilted. The tilt angle is chosen to maximize the array output. Depending upon latitude, season and time of day this angle will vary.

TRACKING ARRAY An array that is mounted on a movable structure that attempts to follow the path of the sun. Some tracking arrays are single axis while others are dual.


VOLTAGE A measure of the force or “push” given the electrons in an electrical circuit; a measure of electrical potential. One volt produces one amp of current when acting against a resistance of one ohm.


WATT Unit of power. Power is the rate of using energy to do work.