GLOSSARY

Interested in solar energy, but not sure what “photovoltaic,” “power purchase agreement,” or “distributed power” refers to? This glossary contains some of the most common terms you’ll encounter in a solar electricity discussion. Want to learn more? Visit the US Department of Energy Efficiency and Renewable Energy online glossary:

Alternating Current (AC): A type of electrical current. The direction of an alternating current is reversed at regular intervals or cycles. In the US, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.

Array: An interconnected system of photovoltaic (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. Also known as a photovoltaic array.

Azimuth Angle: The angle between true south and the point on the horizon directly below the sun.

Balance of System: Represents all components and costs other than the photovoltaic modules/array. It includes design costs, land, site preparation, system installation, support structures, power conditioning, operation and maintenance costs, indirect storage, and other related costs.

Base Load: The average amount of electric power that a utility must supply during any period.

Building Envelope: The separation between the interior and exterior environments of a building. It serves as the outer shell to protect the indoor environment as well as to facilitate its climate control.

Building-Integrated Photovoltaics (BIPV): A term for the design and integration of photovoltaic (PV) technology into the building envelope, typically replacing conventional building materials.

British Thermal Unit (Btu): Amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.

Concentrator: A photovoltaic module that includes optical components such as lenses (Fresnel lens) to direct and concentrate sunlight onto a solar cell of smaller area. Most concentrator arrays must directly face or track the sun. They can increase the power flux of sunlight hundreds of times.

Conductor: Material through which electricity is transmitted, such as an electrical wire, transmission or distribution line.

Conversion Efficiency: See photovoltaic (conversion) efficiency.

Converter: A unit that converts a direct current (dc) voltage to another dc voltage.

Crystalline Silicon: A type of photovoltaic cell made from a slice of singlecrystal silicon or polycrystalline silicon.

Current: The flow of electrical energy (electricity) in a conductor; measured in amperes.

Current at Maximum Power (Imp): The current at which maximum power is available from a module.

Direct Current (DC): A type of electricity transmission and distribution by which electricity flows in one direction through the conductor, usually relatively low voltage and high current. DC must be converted to alternating current in order to be used for typical 120 volt or 220 volt household appliances.

DC-to-DC Converter: Electronic circuit to convert direct current voltages (e.g. photovoltaic module voltage) into other levels (e.g., load voltage). Can be part of a maximum power point tracker.

Distributed Generation: A popular term for localized or on-site power generation.

Distributed Power: Generic term for any power supply located near the point where the power is used. Opposite of central power. See stand-alone systems.

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.

Downtime: Time when the photovoltaic system cannot provide power for the load. Usually expressed in hours per year or the percentage.

Electric Circuit: The path followed by electrons to and from a power source (generator or battery) through an electrical system.

Electric Current: The flow of electrical energy (electricity) in a conductor, measured in amperes.

Electrical grid: An integrated system of electricity distribution, usually covering a large area.

Energy Audit: A survey that shows how much energy is used in a home. The audit will help identify ways to use less energy.

Fixed Tilt Array: A photovoltaic array that does not pivot to follow the track of the Sun in the sky.

Flat-Plate Array: A photovoltaic array that consists of non-concentrating (or flat) modules.

Flat-Plate Module: An arrangement of photovoltaic cells or material mounted on a rigid flat surface with the cells exposed freely to incoming sunlight.

Flat-Plate Photovoltaics: A PV array or module that consists of non-concentrating elements. Flat-plate arrays and modules use direct and diffuse sunlight, but if the array is fixed in position, some portion of the direct sunlight is lost because of oblique sun-angles in relation to the array.

Fresnel Lens: An optical device that focuses light like a magnifying glass. Concentric rings are faced at slightly different angles so that light falling on any ring is focused to the same point.

Full Sun: The amount of power density in sunlight received at the earth's surface at noon on a clear day (about 1,000 Watts/square meter).

General Obligation Bond (GO Bond): Municipal bonds secured by a school district that are backed by the district’s credit and tax revenues.

Gigawatt (GW): A unit of power equal to 1 billion Watts; 1 million kilowatts, or 1,000 megawatts.

Grid: An integrated system of electricity distribution, usually covering a large area.

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

Grid-Interactive System: Same as grid-connected system.

Hybrid System: A solar electric or photovoltaic system that includes other sources of electricity generation, such as wind or diesel generators.

Independent Power Producer (IPP): An entity, which is not a public utility, but which owns facilities to generate electric power for sale to utilities and end users. NUGs may be privately-held facilities, cooperatives such as rural solar or wind energy producers, and non-energy industrial concerns capable of feeding excess energy into the system. Also known as a non-utility generator.

Insolation: A measure of solar radiation (sunlight) energy received on a given surface area in a given time. Insolation is usually expressed as Watts per square meter or Btu per square foot per hour.

Inverter: A device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid.

Junction Box: An enclosure on a photovoltaic (PV) module where PV strings are electrically connected and where protection devices can be located, if necessary.

Kilowatt (kW): A standard unit of electrical power equal to 1000 watts, or to the energy consumption at a rate of 1000 joules per second.

Kilowatt-Hour (kWh): 1,000 thousand watts acting over a period of 1 hour. The kWh is a unit of energy. 1 kWh=3600 kJ.

Life: The period during which a system is capable of operating above a specified performance level.

Life-Cycle Cost: The estimated cost of owning and operating a PV system for the period of its useful life.

Load: The demand on an energy producing system; the energy consumption or requirement of a piece or group of equipment. Usually expressed in terms of amperes or watts in reference to electricity.

Load Circuit: The wire, switches, fuses, etc. that connect the load to the power source.

Load Current (A): The current required by the electrical device.

Megawatt (MW): 1,000 kilowatts, or 1 million watts. Megawatt is the standard measure of electric power plant generating capacity.

Megawatt-Hour: 1,000 kilowatt-hours or 1 million watt-hours.

Module: The smallest planar assembly of solar cells and ancillary parts, such as interconnections, terminals, [and protective devices such as diodes] intended to generate direct current power under unconcentrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate).

Module Derate Factor: A factor that lowers the photovoltaic module current to account for field operating conditions such as dirt accumulation on the module.

National Electrical Code (NEC): Contains guidelines for all types of electrical installations. The 1984 and later editions of the NEC contain Article 690, "Solar Photovoltaic Systems" which should be followed when installing a PV system.

NEMA: National Electrical Manufacturers Association. NEMA is the trade association of choice for the electrical manufacturing industry. Founded in 1926 and headquartered near Washington, D.C., its approximately 450 member companies manufacture products used in the generation, transmission and distribution, control and end-use of electricity.

Net Metering: Allows customers to measure the difference between the electricity they receive from the electric grid and the electricity that is generated on their premises and delivered back to the electric grid. Many utilities offer credits for electricity generated on the customer’s premises which can be used to off-set the customer’s electricity bill from the grid.

Non-Utility Generator (NUG): An entity, which is not a public utility, but which owns facilities to generate electric power for sale to utilities and end users. NUGs may be privately-held facilities, cooperatives such as rural solar or wind energy producers, and non-energy industrial concerns capable of feeding excess energy into the system. Also known as an independent power producer.

One-Axis Tracking: A PV system capable of rotating about one axis.

Operating Point: The current and voltage that a photovoltaic module or array produces when connected to a load. The operating point is dependent on the load or the batteries connected to the output terminals of the array.

Orientation: Placement with respect to the cardinal directions, North, South, East and West.

Packing Factor: The ratio of array area to actual land area or building envelope area for a system; or, the ratio of total solar cell area to the total module area, for a module.

Panel: Often used interchangeably with PV module (especially in one-module systems), but more accurately used to refer to a physically connected collection of modules (i.e., a laminate string of modules used to achieve a required voltage and current).

Peak Demand/Load: The maximum energy demand or load in a specified time period.

Peak Power Current: Amperes produced by a photovoltaic module or array operating at the voltage of the I-V curve that will produce maximum power from the module.

Peak Sun Hours: The equivalent number of hours per day when solar irradiance averages 1,000 w/m2. For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for six hours been 1,000 w/m2.

Peak Watt: A unit used to rate the performance of solar cells, modules, or arrays. The maximum nominal output of a photovoltaic device, in watts (Wp) under standardized test conditions, usually 1,000 watts per square meter of sunlight with other conditions, such as temperature specified.

Photon: A particle of light that acts as an individual unit of energy.

Photovoltaic(s) (PV): Pertaining to the direct conversion of light into electricity.

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

PV Cell: The smallest semiconductor element within a PV module that performs the immediate conversion of light into electrical energy. Also called a solar cell.

PV Conversion Efficiency: The ratio of electric energy that is produced from sunlight by a photovoltaic device.

PV Device: A solid-state electrical device that converts light directly into direct current electricity. Solar photovoltaic devices are made of various semiconductor materials including silicon, cadmium sulfide, cadmium telluride, and gallium arsenide, and in single crystalline, multicrystalline, or amorphous forms.

PV 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.

PV Generator: The total of all PV strings of a PV power supply system, which are electrically interconnected.

PV 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 direct current power under unconcentrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate).

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

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.

Point-Contact Cell: A high efficiency silicon photovoltaic concentrator cell that employs light trapping techniques and point-diffused contacts on the rear surface for current collection.

Power Conversion Efficiency: The ratio of output power to input power of the inverter.

Power Factor (PF): The ratio of actual power being used in a circuit, expressed in watts or kilowatts, to the power that is apparently being drawn from a power source, expressed in volt-amperes or kilovolt-amperes.

Power Purchase Agreement (PPA): A long-term contract (typically 20-25 years) between an independent power producer and its customer. Under the terms of a PPA, the customer agrees to purchase solar energy that is generated from a solar electric system the PPA provider installs on the customer’s facility. The independent power producer pays for the project, maintains and monitors the energy production, and sells the electricity to the customer at a contracted price for the term of the contract.

Projected Area: The net south-facing glazing area projected on a vertical plane.

Renewable Energy Certificates (RECs): A tradable environmental commodity in the United States that can be sold, traded or bartered; the owner of the REC can claim to have purchased renewable energy. A REC represents proof that 1 megawatt-hour (MWh) of electricity was generated from an eligible renewable energy resource. The energy associated with a REC is sold separately and is used by another party; the consumer of a REC receives only a certificate. RECs are also known as Green tags, Renewable Energy Credits, or Tradable Renewable Certificates (TRCs).

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

Solar Energy: Electromagnetic energy transmitted from the sun (solar radiation). The amount that reaches the earth is equal to one billionth of total solar energy generated, or the equivalent of about 420 trillion kilowatt-hours.

Solar Noon: The time of the day, at a specific location, when the sun reaches its highest, apparent point in the sky; equal to true or due, geographic south.

Solar Panel: Often used interchangeably with PV module and PV panel. Solar panel refers to a physically connected collection of modules (i.e., a laminate string of modules used to achieve a required voltage and current).

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

String: A number of photovoltaic modules or panels interconnected electrically in series to produce the operating voltage required by the load.

Thin Film: A layer of semiconductor material, such as copper indium diselenide or gallium arsenide, a few microns or less in thickness, used to make photovoltaic cells.

Thin Film Photovoltaic Module: A photovoltaic module constructed with sequential layers of thin film semiconductor materials. See amorphous silicon.

Tilt Angle: The angle at which a photovoltaic array is set to face the sun relative to a horizontal position. The tilt angle can be set or adjusted to maximize seasonal or annual energy collection.

Tracking Array: A photovoltaic (PV) array that follows the path of the sun to maximize the total sunlight that reaches the PV surface. The two most common orientations are (1) one axis where the array tracks the sun east to west and (2) two-axis tracking where the array points directly at the sun at all times. Tracking arrays use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy.

Two-Axis Tracking: A photovoltaic array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal).

Utility-Interactive Inverter: An inverter that can function only when tied to the utility grid. This inverter uses the prevailing line-voltage frequency on the utility line as a control parameter to ensure that the photovoltaic system's output is fully synchronized with the utility power.

Volt (V): A unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.

Voltage: The amount of electromotive force, measured in volts, that exists between two points.

Watt: The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).

Solar Zenith Angle: The angle between vertical and the direction of the sun. Compare to “Solar Elevation Angle”- the elevation angle of the sun and the horizon.