TERMS & ABBREVIATIONS:

Air mass A measure of how far light travels through the Earth's atmosphere. One air mass, or AM1, is the thickness of the Earth's atmosphere. Air mass zero (AM0) describes solar irradiance in space, where it is unaffected by the atmosphere. The power density of AM1 light is about 100 mW/cmÂ2; the power density of AM0 light about 136 mW/cmÂ2.



Ampere A unit of electrical current. A potential of one volt across a resistance of one ohm causes a current of one ampere (6.25 X 1018 electrons per second) to flow.



Ampere-hour A unit of energy, typically referring to battery capacity. One ampere of current flowing for one hour.



Azimuth The angular measure between due south and the point on the horizon directly below the sun.



Array A number of photovoltaic modules electrically connected to produce a single electrical output.



Angle of incidence The angle between a ray of sunlight striking a surface and a line perpendicular to that surface. Rays perpendicular to a surface have a zero angle of incidence.

Asynchronous (Induction) Motor An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction, rather than by slip rings and commutators as in slip-ring AC motors. It is generally thought these motors are better for electric because of less need ofr heat reduction systems.



Balance-of-Systems (BOS) components All the components in a power system other than the photovoltaic array



Battery Two or more electrochemical cells connected to provide energy storage. Commonly used to designate one cell also.



Blocking diode A diode which prevents reverse current flow in a circuit, commonly used to prevent a battery from discharging through the array at night.



Charge controller The PV system component which controls the battery's state of charge. It may also provides other system control functions.



Charge rate The current applied to a battery to restore its energy capacity. The rate is typically normalized with respect to the battery's full capacity and a designated time period. Thus, the current necessary to nominally charge a 100-ampere-hour battery from zero to full charge in 5 hours (20 amperes) is referred to as the battery's C/5 rate. The term is also applied to discharge rate.



Concentrator array A PV array which uses concentrating devices (reflectors, lenses) to increase the intensity of the sunlight striking the array.



DC Direct current. The type of current provided by a battery or solar cell, which flows in one direction.



Depth of discharge A measure of how much energy has been withdrawn from a battery, expressed as a percentage of full capacity. A 100 Ah battery from which 30 Ah has been withdrawn has undergone a 30% depth of discharge (DOD). This term is the inverse of state of charge (SOC); the example battery would be at 70% SOC.



Diffuse radiation The sunlight received indirectly, as a result of scattering due to clouds, fog, dust, moisture vapor or other substances in the atmosphere.



Direct radiation Sunlight received directly, which has traveled in a straight path from the sun, also referred to as beam radiation.



Efficiency With respect to solar cells, the percentage of light energy that is converted to electricity by the cell. Depending on cell technology and production technique, this ranges from as low as 5% to as high as 30%.



Elevation (solar) The sun's angle above the horizon.



Equalizing charge A controlled overcharge of a battery bank for the purpose of restoring equality of charge in all cells.

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Finishing charge That part of the charging process which restores the final segment of a battery's charge, roughly between 90% and 100% SOC.

Flat-plate array A PV array which does not use concentration.

Frequency The rate at which a periodic event occurs. In electricity, the rate at which current reverses direction in an alternating current system. In the US, alternating current systems use a frequency of 60 cycles per second (60 Hz); in Europe, the standard is 50 Hz.

Global radiation Total solar radiant energy impinging on a surface, equal to the sum of direct and diffuse radiation.

Grid-connected A power system interconnected with the grid (or mains) of the local electric utility. Also referred to as utility-interactive.

Hole A vacancy in a crystalline structure, which would be filled by an electron if the structure were electrically perfect.

Hybrid system A power system consisting of two or more energy sources (e.g., a PV array and a wind generator.)

Induction Motor See Asynchronous Motor

Insolation The solar energy received at a place over a given period. May be expressed as sun-hours per day, langleys per hour, watts per square meter per hour, or any number of other units.

Inverter A device which converts DC electricity to AC.

Isolation diode A diode which prevents one segment of a PV array from interacting with another array segment. Usually used to prevent array energy from flowing backwards through a sub-voltage series string. May also serve the function of blocking diode.

I-V curve A current/voltage curve, which expresses the possible combinations of current and voltage output of a photovoltaic device.

Maximum power Also referred to as peak power. The point on a device's I-V curve where the product of I and V (Pmax, measured in watts) is maximized. The points on the I and V scales which describe this curve point are named Imp (current @ max power) and Vmp (voltage @ max power.)

Module A number of solar cells electrically connected, protected from environmental stresses, self-contained and not subdividable, providing a single electrical output.

NOCT Nominal Operating Cell Temperature, the temperature at which cells in a module operate under Standard Operating Conditions (SOC), which are: irradiance of 0.8 kW/m2, 20ºC ambient temperature, and average windspeed of 1 m./s, with the wind oriented parallel to the plane of the array, and all sides of the array fully exposed to the wind.

Open-circuit voltage Abbreviated Voc,, refers to a PV device's voltage potential when it is providing no current.

Parallel connection Electrical connection where the positive terminals of a number of devices are connected together, as are their negative terminals. The output voltage of the paralleled devices is equal to the average of the devices, and the total current is the sum of the current of all the devices.

p-n junction The junction at the interface between two differently doped layers of semiconductor material, one layer doped with a positive-type dopant, the other layer with a negative-type dopant. An electrical field is established at the p-n junction which gives direction to the flow of light-stimulated electrons.

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Self-discharge Batteries lose charge even when not in use. For many batteries, self-discharge rate increases as the battery ages.

Series connection Electrical connection where the positive terminal of one device is attached to the negative terminal of the next in a series string; in this connection, the string voltage is the sum of the device voltages and the string current is limited to the current of the least productive device in the string.

Short-circuit current Abbreviated Isc, refers to a PV device's current output when short-circuited.

Solar panel A group of photovoltaic modules mechanically mounted on a single frame.

Solar spectrum The total distribution of electromagnetic radiant energy over the band of wavelengths present in solar radiation. The total energy received on a given surface, and how that energy is distributed among various wavelengths, depends on how much of the Earth's atmosphere light has traversed.

Standard operating conditions Abbreviated SOC, a set of reference PV device measurement conditions consisting of irradiance of 0.8 kW/m2, 20ºC ambient temperature, and average windspeed of 1 m./s, with the wind oriented parallel to the plane of the array, and all sides of the array fully exposed to the wind.

Standard test conditions Abbreviated STC, a set of reference PV device measurement conditions consisting of irradiance of 1 kW/m2, AM 1.5, and 25ºC cell temperature.

Standalone system A power system not connected to the utility grid (mains.) Sometimes referred to as an autonomous system.

Standoff mount A mounting system which supports a PV array above a roof surface.

State of charge Abbreviated SOC, the percentage of energy in a battery referenced to its nominal full capacity.

Sulfation The formation of lead sulfate crystals on the plates of a lead-acid battery. Normally used to refer to large sulfate crystals, rather than small crystals formed in normal battery operation, formed as a result of temperature cycling while the battery is in a partially charged state.

Thin film cell A PV cell formed by depositing thin layers of conductive and semiconductive materials, usually using a chemical vapor deposition (CVD) process. Also referred to as amorphous cells because they have no crystalline structure, such cells use less material than cells sawn from crystalline ingots.

Two-axis tracking A tracking system which follows the sun's azimuth and elevation.

Utility-interactive A power system which interacts with the utility grid (mains), taking power from the grid to satisfy its loads as necessary, and returning power to the grid when not required by the loads.

Voltage Measured in volts (V), the electrical potential between two points. One volt of potential causes one ampere of current to flow through a resistance of one ohm. The open-circuit voltage of a silicon solar cell is about half a volt; the operating voltage of a lead-acid cell is about two volts.

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