The flow of electricity that constantly changes direction between positive and negative sides. Almost all power produced by electric utilities in the United States moves in current that shifts direction at a rate of 60 times per second.

The unit of measure that indicates how much electricity flows through a conductor. It is like using cubic feet per second to measure the flow of water. For example, a 1,200-watt, 120-volt hair dryer pulls 10 amperes of electricity current. (amps = watts/volts)

A thin coating of a material, which reduces the light reflection and increases light transmission, applied to a photovoltaic cell surface or to a thin sheet of glass protecting the photovoltaic cell surface.

A collection of electrically connected photovoltaic (PV) modules.

The energy demand for a given location over a period of time. For example, the number of kilowatt-hours used in a 24-hour period, divided by 24, tells the average demand for that location in that time period.

The amount of money an electric utility would need to spend for the next increment of electric generation to produce or purchase.

Horizontal angle measured clockwise from true north; 180° is true south.

Batteries are often sold with a PV system. The primary purpose is to store the electricity not immediately used, which could be used at some later time. With net metering, the value of batteries is less because the utility grid basically acts as a storage facility. For a reliable generation system that can function independent of the utility grid, however, batteries may be a viable component to the total system. Back-up generators may be included in a system to provide power when the PV system is not operating, and are generally included when systems are not grid connected. Neither batteries nor generators are eligible for rebate money.

The total number of ampere-hours that can be withdrawn from a fully charged battery.

Building Integrated Photovoltaics; A term for the design and integration of PV into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades, replacing view glass, spandrel glass, or other facade material; into semitransparent skylight systems; into roofing systems, into shading "eyebrows" over windows; or other building envelope systems.

A diode used to restrict or block reverse current from flowing backward through a module. Alternatively, diode connected in series to a PV string; it protects its modules from a reverse power flow and, thus, against the risk of thermal destruction of solar cells.

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, 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.

A device that produces electricity from light. Cells are the building block for modules.

A device that controls the charging rate and/or state of charge for batteries.

One or more conductors through which electricity flows.

A type of PV cell made from a single crystal or polycrystalline slice of silicon.

The flow of electric charge in a conductor between two points having a difference in potential (voltage).

The amount of power your site uses. Load may be expressed in kilowatts (capacity) or kilowatt-hours (energy). A site's peak kilowatts generally refers to when electric demand requirements are highest.

The discharge and subsequent charge of a battery.

Type of battery that can be discharged to a large fraction of capacity many times without damaging the battery.

The level at which electricity (or natural gas) is delivered to end-users at a given point in time. Electric demand is measured in kilowatts.

The flow of electricity that flows continuously in one direction.

The ratio of output power (or energy) to input power (or energy). Expressed in percent.

A flow of electrons; electricity.

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

A PV array set in at a fixed angle with respect to horizontal.

The number of cycles through which an alternating current moves in each second. Standard electric utility frequency in the United States is 60 cycles per second, or 60 hertz.

A crystalline, high-efficiency semiconductor/photovoltaic material.

Gas by-products, primarily hydrogen, produced when charging a battery. Also, termed out-gassing.

Lead-acid battery in which the electrolyte is composed of a silica gel matrix.

The electricity transmission and distribution system that links power plants to customers through high-power transmission line service.

A PV system in which the PV array acts like a central generating plant, supplying power to the grid.

The unit of electromagnetic wave frequency that is equal to one cycle per second.

The solar radiation incident on an area over time. Equivalent to energy and usually expressed in kilowatt-hours per square meter.

The linkage of transmission lines between two utilities, or between a utility and an end-user, enabling power to be moved in either direction.

Converts DC power from the PV array/battery to ACc power compatible with the utility grid and local AC loads.

The solar power incident on a surface. Usually expressed in kilowatts per square meter. Irradiance multiplied by time equals Insolation.

An enclosure where multiple PV modules are electrically connected and where protection devices can be located, if necessary.

One thousand watts. A unit of measure of the amount of electricity needed to operate given equipment. For example, one kW is enough power to illuminate 10 light bulbs at 100 watts each. (volts x amps = watts)

The amount of kW produced over a period of time, or one kilowatt of electricity supplied for one hour. A unit of energy.  Power multiplied by time equals energy.   For example, a one kW PV system, if operating at full capacity for 5 hours will produce 5 kWh of electricity during that time.

The amount of electric power used by any electrical unit or appliance at any given time.

A sealed battery to which water cannot be added to maintain electrolyte level.

One thousand kilowatts or one million watts. One megawatt is enough to power 1,000 average California homes per day.

A device that measures levels and volumes of customers' electricity and gas use.

An integral, encapsulated unit containing a number of PV cells.  A group of photovoltaic cells combined in series and/or parallel and encapsulated in an environmentally protective laminate. The smallest replaceable unit in a PV array, modules are the building blocks for arrays.

Equipment/apparatus used to fasten PV modules to the roof or ground.

Maximum Power Point; 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; Means of a power conditioning unit that automatically operates the PV-generator at its MPP under all conditions.

National Electrical Manufacturers Association, the organization which sets standards for electrical products and some non-electronic products like junctions boxes.

Net Metering is a billing arrangement between your utility and its customers who generate their own electricity with photovoltaic or wind systems, by which the excess electricity produced using your own generating equipment is fed back onto the utility grid and is deducted from your electric bill.

A system capable of rotating about one axis.

The maximum voltage produced by an illuminated photovoltaic cell, module, or array with no load connected. This value will increase as the temperature of the PV material decreases.

The highest electrical demand within a particular period of time.

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.

The amount of power a photovoltaic module will produce at standard test conditions (normally 1000 W/m2 and 25° cell temperature).

A particle of light that acts as an individual unit of energy. Its energy depends on wavelength.

Capable of producing a voltage when exposed to radiant energy, especially light.

A group of photovoltaic modules.

A device that produces electricity from light. Cells are the building block for modules.

An integral, encapsulated unit containing a number of PV cells.  A group of photovoltaic cells combined in series and/or parallel and encapsulated in an environmentally protective laminate. The smallest replaceable unit in a PV array, modules are the building blocks for arrays.

An installation of PV modules and other components designed to produce power from sunlight and meet the power demand for a designated load.

A material used to make PV cells which consist of many crystals as contrasted with single crystal silicon.

An instrument used for measuring global solar irradiance.

The current output of a PV module measured at standard test conditions of 1,000 w/m2 and 25°C cell temperature.

The current produced by an illuminated PV cell, module, or array when its output terminals are shorted.

The most common semiconductor material used in making photovoltaic devices; 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.

Material with a single crystalline formation. Many PV cells are made from single crystal silicon.

A process that heats water directly using the sun's radiant energy.

Conditions under which a module is typically tested in a laboratory: (1) Irradiance intensity of 1000 W/m2, AM1.5 solar reference spectrum, and (3) a cell (module) temperature of 25°C, plus or minus 2°C.

Storage refers to saving surplus electricity produced by a PV system. Generally batteries are used as storage devices.

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

The angle of inclination of a solar collector measured from the horizontal.

Structure that houses PV modules and that can automatically follow the sun across the sky throughout the day to maximize output.

A PV array capable of rotating independently about two axes (e.g., vertical and horizontal).

The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.

The interconnection of electricity generation plants through the transmission and distribution lines to customers. The grid also refers to the interconnection of utilities through the electric transmission and distribution systems.

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.

The amount of force required to drive a steady current of one ampere through a resistance of one ohm. Electrical systems of most homes and offices use 120 volts. (Volts = watts/amps)

Electric measurement of power at one point in time, as capacity or demand. For example, light bulbs are classified by wattage. (1000 watts = 1 kilowatt)