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Absorber — In a photovoltaic device, the material that readily absorbs photons to generate charge carriers (free electrons or holes.)

ACSee alternating current.  

Activated shelf life – The period of time, at a specified temperature, that a charged battery can be stored before its capacity falls to an unusable level.

All-of-the-above energy policyAn all-of-the-above energy strategy promotes using all sources of energy, from oil and natural gas, to coal, to solar to wind to nuclear. (6)

Alternating current (AC) – A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles 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.

Ampere-hour (Ah/AH) – A measure of the flow of current (in amperes) over one hour; 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).

Annual solar savings – The annual solar savings of a solar building is the energy savings attributable to a solar feature relative to the energy requirements of a non-solar building.

Antireflection coating – A thin coating of a material applied to a solar cell surface that reduces the light reflection and increases light transmission.

Array current – The electrical current produced by a photovoltaic array when it is exposed to sunlight.

Availability – The quality or condition of a photovoltaic system being available to provide power to a load. Usually measured in hours per year. One minus availability equals downtime.

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 related costs.

Balancing areaA metered segment of the power system, maintained by a balancing area authority, that ensures the total of all electrical generation equals the total of all system loads.

Base load The average amount of electric power that a utility must supply in any period.

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 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 cut-off voltage

Battery capacity The maximum total electrical charge, expressed in ampere-hours, which a battery can deliver to a load under a specific set of conditions.

Boron (B) The chemical element commonly used as the dopant in photovoltaic device or cell material.

British thermal unit (Btu) The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.


Cadmium (Cd) A chemical element used in making certain types of solar cells and batteries.

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). See also photovoltaic (PV) cell.  

Charge The process of adding electrical energy to a battery.

Concentrating photovoltaics (CPV) — A solar technology that uses lenses or mirrors to concentrate sunlight onto high-efficiency solar cells.

Concentrating solar power (CSP) — A solar technology that use mirrors to reflect and concentrate sunlight onto receivers that convert solar energy to heat. This thermal energy is then used to produce electricity with a steam turbine or heat engine driving a generator.

CurrentSee electric current.  

Cycle — The discharge and subsequent charge of a battery.


Days of storage — The number of consecutive days the stand-alone system will meet a defined load without solar energy input. This term is related to system availability.

DCSee direct current.

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. 

Deep-cycle battery — A battery with large plates that can withstand many discharges to a low state-of-charge.

Demand response – The process of using voluntary load reductions during peak hours.

Diode – An electronic device that allows current to flow in one direction only. See also blocking diode and bypass diode.

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. To be used for typical 120-volt or 220-volt household appliances, DC must be converted to alternating current, its opposite.

Distributed energy resources (DER) – A variety of small, modular power-generating technologies that can be combined with energy management and storage systems and used to improve the operation of the electricity delivery system, whether or not those technologies are connected to an electricity grid.

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 also stand-alone systems.

Dopant — A chemical element (impurity) added in small amounts to an otherwise pure semiconductor material to modify the electrical properties of the material. An n-dopant introduces more electrons. A p-dopant creates electron vacancies (holes). 


Electric circuit – The path followed by electrons from a power source (generator or battery), through an electrical system, and returning to the source.

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.

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

Electron – An elementary particle of an atom with a negative electrical charge and a mass of 1/1837 of a proton; electrons surround the positively charged nucleus of an atom and determine the chemical properties of an atom. The movement of electrons in an electrical conductor constitutes an electric current. 

Electron volt (eV) – The amount of kinetic energy gained by an electron when accelerated through an electric potential difference of 1 Volt; equivalent to 1.603 x 10^-19; a unit of energy or work.

Energy – The capability of doing work; different forms of energy can be converted to other forms, but the total amount of energy remains the same.

Energy audit – A survey that shows how much energy used in a home, which helps find ways to use less energy.

Equinox – The two times of the year when the sun crosses the equator and night and day are of equal length; occurring around March 20 or 21 (spring equinox) and September 22 or 23 (fall equinox).

ERCOT – The Electric Reliability Council of Texas (ERCOT) manages the flow of electric power to more than 26 million Texas customers – representing about 90 percent of the state’s electric load. As the independent system operator for the region, ERCOT schedules power on an electric grid that connects more than 46,500 miles of transmission lines and 650+ generation units. It also performs financial settlement for the competitive wholesale bulk-power market and administers retail switching for 8 million premises in competitive choice areas. ERCOT is a membership-based 501(c)(4) nonprofit corporation, governed by a board of directors and subject to oversight by the Public Utility Commission of Texas and the Texas Legislature. Its members include consumers, cooperatives, generators, power marketers, retail electric providers, investor-owned electric utilities, transmission and distribution providers and municipally owned electric utilities.


Frequency – The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).

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).


Gallium (Ga) – A chemical element, metallic in nature, used in making certain kinds of solar cells and semiconductor devices.

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

GridSee electrical grid.

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.

Grid parity – grid parity is when an alternative form of energy (renewable sources like solar and wind) generates power at a levelized cost of electricity that’s equal to or less than the price of buying power from the electric grid. In other words, grid parity is the point when the cost of the alternative energy becomes equal to or less than electricity from conventional energy forms like fossil fuels.


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


Incident light – Light that shines onto the face of a solar cell or module. 

Independent system operator (ISO) – The entity responsible for maintaining system balance, reliability, and electricity market operation.

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

Ion – An electrically charged atom or group of atoms that has lost or gained electron; a loss makes the resulting particle positively charged; a gain makes the particle negatively charged.

Irradiance – The direct, diffuse, and reflected solar radiation that strikes a surface. Usually expressed in kilowatts per square meter. Irradiance multiplied by time equals insulation. 

ISPRA guidelines – Guidelines for the assessment of photovoltaic power plants, published by the Joint Research Centre of the Commission of the European Communities, Ispra, Italy.


Joule – A metric unit of energy or work; 1 joule per second equals 1 watt or 0.737 foot-pounds; 1 Btu equals 1,055 joules.


Kilowatt (kW) – A standard unit of electrical power equal to 1000 watt, 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.


Levelized cost of energy (LCOE) – The cost of energy of a solar system that is based on the system’s installed price, its total lifetime cost, and its lifetime electricity production.

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

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 forecast – Predictions of future demand. For normal operations, daily and weekly forecasts of the hour-by-hour demand are used to help develop generation schedules to ensure that sufficient quantities and types of generation are available when needed.


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

Megawatt-hour – 1,000 kilowatt-hours or 1 million watt-hours.

ModuleSee photovoltaic (PV) 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.

National Electrical Manufacturers Association (NEMA) – This organization sets standards for some non-electronic products like junction boxes.

NECSee National Electrical Code.

NEMASee National Electrical Manufacturers Association.


Off grid – Completely disconnected from the electricity grid, with no access to utility-generated electricity. Homes that go off grid need to generate all of their electricity on-site. (3)

Orientation – Placement with respect to the cardinal directions, N, S, E, W; azimuth is the measure of orientation from north.

Overcharge – Forcing current into a fully charged battery. The battery will be damaged if overcharged for a long period.


PanelSee photovoltaic (PV) panel.

Peak demand/load – The maximum energy demand or load in a specified time period.

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.

Photovoltaic (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.

Photovoltaic (PV) cell – The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (direct current voltage and current).  Also called a solar cell.

Power – The amount of electrical energy available for doing work, measured in horsepower, Watts, or Btu per hour.

Power density – The ratio of the power available from a battery to its mass (W/kg) or volume (W/l).

PVSee photovoltaic(s).


Quad – One quadrillion Btu (1,000,000,000,000,000 Btu).


Ramp – A change in generation output.

Ramp rate – The ability of a generating unit to change its output over some unit of time, often measured in MW/min.

Reserve capacity – The amount of generating capacity a central power system must maintain to meet peak loads.


Satellite power system (SPS) – Concept for providing large amounts of electricity for use on the Earth from one or more satellites in geosynchronous Earth orbit. A very large array of solar cells on each satellite would provide electricity, which would be converted to microwave energy and beamed to a receiving antenna on the ground. There, it would be reconverted into electricity and distributed the same as any other centrally generated power, through a grid.

Scheduling – The general practice of ensuring that a generator is committed and available when needed. It also can refer to scheduling of imports or exports of energy into or out of a balancing area. 

Semiconductor – Any material that has a limited capacity for conducting an electric current. Certain semiconductors, including silicon, gallium arsenide, copper indium diselenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process.

Shelf life of batteries – The length of time, under specified conditions, that a battery can be stored so that it keeps its guaranteed capacity.

Smart grid – An intelligent electric power system that regulates the two-way flow of electricity and information between power plants and consumers to control grid activity.

Solar cellSee photovoltaic (PV) cell.

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.

Solar panelSee photovoltaic (PV) panel.

Solar panel efficiency – Represents how well a solar panel converts sunlight into electricity. Most solar panels have 14 to 16 percent efficiency; high-efficiency panels are rated just above 20 percent.

State-of-charge (SOC) – The available capacity remaining in the battery, expressed as a percentage of the rated capacity.

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.

Sub-hourly energy markets – Electricity markets that operate on time steps of 5 minutes. Approximately 60% of all electricity in the United States is currently traded in sub-hourly markets, running at 5-minute intervals so that maximum flexibility can be obtained from the generation fleet.

Superconductivity – The abrupt and large increase in electrical conductivity exhibited by some metals as the temperature approaches absolute zero.

Superstrate – The covering on the sunny side of a photovoltaic (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 maximum power, usually 3-5 times the rated power, that can be provided over a short time.

System availability – The percentage of time (usually expressed in hours per year) when a photovoltaic system will be able to fully meet the load demand.

System storageSee battery capacity.


312 agreement – A tax abatement is a local agreement between a taxpayer and a taxing unit that exempts all or part of the increase in the value of the real property and/or tangible personal property from taxation for a period not to exceed 10 years. Tax abatements are an economic development tool available to cities, counties and special districts to attract new industries and to encourage the retention and development of existing businesses through property tax exemptions or reductions. School districts may not enter into abatement agreements.

313 agreement – Stipulated by the Texas Economic Development Act, an appraised value limitation is an agreement in which a taxpayer agrees to build or install property and create jobs in exchange for a 10-year limitation on the taxable property value for school district maintenance and operations tax (M&O) purposes. The minimum limitation value varies by school district.

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.

Transformer – An electromagnetic device that changes the voltage of alternating current electricity.


Ultraviolet – Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.

Utility-scale solar facility – a utility-scale solar facility is one that generates solar power and feeds it into the grid, supplying a utility with energy.


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.

Voltage at maximum power (Vmp) – The voltage at which maximum power is available from a photovoltaic module. 


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).




Zenith angle – the angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).

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