Natural Power Solutions, LLC

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Glossary of Terms



Absorbers — A dark colored material or surface that soaks up the heat in a solar collector.
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.
Activation Voltage(s) — The voltage(s) at which a charge controller will take action to protect the batteries.
Adjustable Set Point — A feature allowing the user to adjust the voltage levels at which a charge controller will become active.
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.
Air mass — The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness).
Ambient Temperature — The temperature of the surrounding area.
Ampere (amp) — A unit of electrical current or rate of flow of electrons.
Ampere-Hour (Ah/AH) — A measure of the flow of current (in >amperes) over one hour; used to measure battery capacity.
Amorphous solar panels - Amorphous solar panels are also referred to as "thin film" solar panels.  In these types of panels the silicon is spread directly on large plates, usually of something like stainless steel. The thin film type of solar cells can also be spread on to more flexible plastic materials to make very flexible solar panels.  These types of solar cells are much cheaper but also much less efficient than mono crystalline or poly-crystalline solar panels.  Therefore in order to provide as many watts as the other types of solar panels they must be much bigger in size.  However, because they can be put on to flexible backings they have proven very valuable in certain types of applications where flexibility is more critical than power.  For example, these types of solar panels are often used in portable products such as solar backpacks and solar bags. Overall efficiency on average is about 5-6%.
Angle of Incidence — The angle that a ray of sun makes with a line perpendicular to the surface. For example, a surface that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°.
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.
Anode — The positive electrode in an electrochemical cell (battery). Also, the earth or ground in a cathodic protection system. Also, the positive terminal of a diode.
Antireflection Coating — A thin coating of a material applied to a solar cell surface that reduces the light reflection and increases light transmission.
Array — see photovoltaic (PV) array.
Array Current — The electrical current produced by a photovoltaic array when it is exposed to sunlight.
Array Operating Voltage — The voltage produced by a photovoltaic array when exposed to sunlight and connected to a load.
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.
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.
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 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. Life may be measured in cycles and/or years, depending on the type of service for which the cell or battery is intended.
Bifacial Mono-crystalline solar panels - A new type of solar panel has recently emerged on the market which uses mono-crystalline solar cells but which has glass on both sides so that it can collect energy from both sides of the solar panel.  By collecting light from both sides the bifacial panels have higher efficiency for about the same cost.  Efficiency levels up to 20% have been reported for these types of panels.  Typically these panels are installed in a pole mounted solar array so that ambient light can reach the panel from both the front and the back.  They can also be effective if roof mounted on a roof that has a white matt or has been painted white to allow light to reflect on to the back of the panel.  Because these panels are fairly new there is not a lot of information yet on their durability but most are warranted for 20-25 years, the same as for traditional panels.  Bi-facial panels are a particularly attractive solution for pole mounted systems since a given pole mount can usually only hold 9-12 solar panels.  By using more efficient panels the cost tradeoff of the panels versus the cost of the tracking system is improved. Currently Sanyo is the leading manufacturer of bifacial solar panels.
Btu (British thermal unit) — The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.

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.
Cell Junction — The area of immediate contact between two layers (positive and negative) of a photovoltaic cell. The junction lies at the center of the cell barrier or depletion zone.
Charge — The process of adding electrical energy to a battery.
Charge Controller — A component of a photovoltaic system that controls the flow of current to and from the battery to protect it from over-charge and over-discharge. The charge controller may also indicate the system operational status.
Charge Factor — A number representing the time in hours during which a battery can be charged at a constant current without damage to the battery. Usually expressed in relation to the total battery capacity, i.e., C/5 indicates a charge factor of 5 hours.
Cloud Enhancement — The increase in solar intensity caused by reflected irradiance from nearby clouds.
Combined Collector — A photovoltaic device or module that provides useful heat energy in addition to electricity.
Concentrating Photovoltaic Solar Panels -  These types of panels employ a lens or mirror to concentrate the sun's energy on to the individual solar cells. In theory these types of panels will be more efficient because by concentrating the sun's energy fewer solar cells are needed to create the same amount of energy. Many of the concentrating panels use a type of plastic lens, called a Fresnell lens, to concentrate the sun's energy. Another type of concentrating solar panel called a Heliotube uses a series of troughs which track the movement of the sun to provide greater solar exposure to the solar cells.  Concentrating solar panels reduce the amount of photovoltaics needed to produce electricity, and also reduce the amount of space needed for a photovoltaic installation. Their main disadvantage is that they depend solely on direct light to produce electricity, while stand-alone photovoltaic panels can use both direct and diffuse light. Many regions do not receive enough direct light throughout the year for these systems to make these types of panels practical. Another disadvantage is their complexity of their construction, which makes these systems more difficult to build and install than conventional PV panels. Concentrating panels are also considerably heavier than conventional PV panels and have a number of moving parts which makes them more susceptible to failure than conventional panels. These types of panels are not widely used in residential solar PV systems.
Concentrator — A photovoltaic module, which 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 — The material through which electricity is transmitted, such as an electrical wire, or transmission or distribution line.
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 single-crystal silicon or polycrystalline silicon.
Cutoff Voltage — The voltage levels (activation) at which the charge controller disconnects the photovoltaic array from the battery or the load from the battery.
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.
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.
Deep Discharge — Discharging a battery to 20% or less of its full charge capacity.
Diffuse Insolation — Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct insolation.
Diffuse Radiation — Radiation received from the sun after reflection and scattering by the atmosphere and ground.
Direct Beam Radiation — Radiation received by direct solar rays. Measured by a pyrheliometer with a solar aperture of 5.7° to transcribe the solar disc.
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.
Direct Insolation — Sunlight falling directly upon a collector. Opposite of diffuse insolation.
Discharge — The withdrawal of electrical energy from a battery.
Discharge Rate — The rate, usually expressed in amperes or time, at which electrical current is taken from the battery.
Disconnect — Switch gear used to connect or disconnect components in a photovoltaic system.
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.
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
Downtime — Time when the photovoltaic system cannot provide power for the load. Usually expressed in hours per year or that percentage.
Duty Cycle — The ratio of active time to total time. Used to describe the operating regime of appliances or loads in photovoltaic systems.
Duty Rating — The amount of time an inverter (power conditioning unit) can produce at full rated power.

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.
Electrochemical Cell — A device containing two conducting electrodes, one positive and the other negative, made of dissimilar materials (usually metals) that are immersed in a chemical solution electrolyte) that transmits positive ions from the negative to the positive electrode and thus forms an electrical charge. One or more cells constitute a battery.
Electrode — A conductor that is brought in conducting contact with a ground.
Electrolyte — A nonmetallic (liquid or solid) conductor that carries current by the movement of ions (instead of electrons) with the liberation of matter at the electrodes of an electrochemical cell.
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.
Energy Density — The ratio of available energy per pound; usually used to compare storage batteries.
Energy Levels — The energy represented by an electron in the band model of a substance.
Equalization — The process of restoring all cells in a battery to an equal 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.
Equinox — The two times of the year when the sun crosses the equator and night and day are of equal length; usually occurs on March 21st (spring equinox) and September 23 (fall equinox).

Fixed Tilt Array — A photovoltaic array set in at a fixed angle with respect to horizontal.
Flat-Plate Module — An arrangement of photovoltaic cells or material mounted on a rigid flat surface with the cells exposed freely to incoming sunlight.
Float Charge — The voltage required to counteract the self-discharge of the battery at a certain temperature.
Float Life — The number of years that a battery can keep its stated capacity when it is kept at float charge.
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.
Frequency — The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).
Frequency Regulation — This indicates the variability in the output frequency. Some loads will switch off or not operate properly if frequency variations exceed 1%.
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).

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.
Gassing Current — The portion of charge current that goes into electrolytical production of hydrogen and oxygen from the electrolytic liquid. This current increases with increasing voltage and temperature.
Gigawatt (GW) — A unit of power equal to 1 billion Watts; 1 million kilowatts, or 1,000 megawatts.
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 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.

High Voltage Disconnect
 — The voltage at which a charge controller will disconnect the photovoltaic array from the batteries to prevent overcharging.
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.
Infrared Radiation — Electromagnetic radiation whose wavelengths lie in the range from 0.75 micrometer to 1000 micrometers; invisible long wavelength radiation (heat) capable of producing a thermal or photovoltaic effect, though less effective than visible light.
Interconnect — A conductor within a module or other means of connection that provides an electrical interconnection between the solar cells.
Inverter — A device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid.
Irradiance — The direct, diffuse, and reflected solar radiation that strikes a surface. Usually expressed in kilowatts per square meter. Irradiance multiplied by time equals insolation.

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.
Junction — A region of transition between semiconductor layers, such as a p/n junction, which goes from a region that has a high concentration of acceptors (p-type) to one that has a high concentration of donors (n-type).
Junction Box — A photovoltaic (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.

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.

Langley (L) — Unit of solar irradiance. One gram calorie per square centimeter. 1 L = 85.93 kwh/m2.
Lead-Acid Battery — A general category that includes batteries with plates made of pure lead, lead-antimony, or lead-calcium immersed in an acid electrolyte.
Life-Cycle Cost — The estimated cost of owning and operating a photovoltaic system for the period of its useful life.
Light Trapping — The trapping of light inside a semiconductor material by refracting and reflecting the light at critical angles; trapped light will travel further in the material, greatly increasing the probability of absorption and hence of producing charge carriers.
Line-Commutated Inverter — An inverter that is tied to a power grid or line. The commutation of power (conversion from direct current to alternating current) is controlled by the power line, so that, if there is a failure in the power grid, the photovoltaic system cannot feed power into the line.
Liquid Electrolyte Battery — A battery containing a liquid solution of acid and water. Distilled water may be added to these batteries to replenish the electrolyte as necessary. Also called a flooded battery because the plates are covered with the electrolyte.
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.
Load Resistance — The resistance presented by the load.
Low Voltage Cutoff (LVC) — The voltage level at which a charge controller will disconnect the load from the battery.
Low Voltage Warning — A warning buzzer or light that indicates the low battery voltage set point has been reached.

Maintenance-Free Battery — A sealed battery to which water cannot be added to maintain electrolyte level.
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.
Module Derate Factor — A factor that lowers the photovoltaic module current to account for field operating conditions such as dirt accumulation on the module.
These types of solar panels uses solar cells which are made from a very pure single large crystal, cut from ingots. They are the most efficient type of solar panels but are also the most expensive. Their performance is somewhat better in low light conditions (but not as good as some advertising hype would have you believe).  Overall efficiency on average is about 12-15%. Most panels of this type are warranted for 20-25 years. They are usually blue-grey in color and have a fairly uniform consistency.  Some of the major manufacturers of mono-crystalline solar panels are Sharp, Kyocera and BP.
Maximum Power Point Tracker is a high efficiency DC to DC converter which functions as an optimal electrical load for a photovoltaic cell, most commonly for a solar panel or array, and converts the power to a voltage or current level which is more suitable to whatever load the system is designed to drive.  The benefits of MPPT regulators are greatest during cold weather, on cloudy or hazy days or when the battery is deeply discharged. Solar MPPTs can also be used to drive motors directly from solar panels. The benefits seen are huge, especially if the motor load is continuously changing. This is due to the fact that the AC impedance across the motor is related to the motor's speed. The MPPT will switch the power to match the varying resistance.
Multi-Stage Controller — A charging controller unit that allows different charging currents as the battery nears full state_of_charge.


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..
Nickel Cadmium Battery — A battery containing nickel and cadmium plates and an alkaline electrolyte.
Nominal Voltage — A reference voltage used to describe batteries, modules, or systems (i.e., a 12-volt or 24-volt battery, module, or system).

Ohm — A measure of the electrical resistance of a material equal to the resistance of a circuit in which the potential difference of 1 volt produces a current of 1 ampere.
One-Axis Tracking — A system capable of rotating about one axis.
Open-Circuit Voltage (Voc) — The maximum possible voltage across a photovoltaic cell; the voltage across the cell in sunlight when no current is flowing.
Orientation — Placement with respect to the cardinal directions, N, S, E, W.
Overcharge — Forcing current into a fully charged battery. The battery will be damaged if overcharged for a long period.

Parallel Circuit — Components connected in parallel are connected so the same voltage is applied to each component.  In a parallel circuit, the voltage across each of the components is the same, and the total current is the sum of the currents through all the components.
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.
Photoelectric Cell — A device for measuring light intensity that works by converting light falling on, or reach it, to electricity, and then measuring the current; used in photometers.
Photo electrochemical Cell — A type of photovoltaic device in which the electricity induced in the cell is used immediately within the cell to produce a chemical, such as hydrogen, which can then be withdrawn for use.
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.
Photovoltaic (PV) Conversion Efficiency — The ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device.
Photovoltaic (PV) Device — A solid-state electrical device that converts light directly into direct current electricity of voltage-current characteristics that are a function of the characteristics of the light source and the materials in and design of the device.
Photovoltaic (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.
Photovoltaic (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.
Photovoltaic (PV) 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.
Photovoltaic (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.
Plates — A metal plate, usually lead or lead compound, immersed in the electrolyte in a battery.
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.
Poly-crystalline Block - With most poly-crystalline solar panels the silicon in the solar cells is cast from large blocks of silicon which may contain many small crystals. Some manufacturers use a slightly different approach for creating poly-crystalline solar cells.  Currently, poly-crystalline solar panels are the most common.  They are slightly less efficient than single crystal, but once set into a frame with 35 or so other cells, the actual difference in watts per square foot is not much.  Poly-crystalline cells look somewhat like shattered glass and have a dark blue to almost black color. Overall efficiency on average is about 11-13%.
Poly-crystalline String Ribbon
String ribbon photovoltaic’s use a variation on the polycrystalline production process, using the same molten silicon but slowly drawing a thin strip of crystalline silicon out of the molten form between two strings. These strips of photovoltaic material are then assembled in a panel with the same metal conductor strips attaching each strip to the electrical current. This technology saves on costs over standard polycrystalline panels as it eliminates the sawing process for producing wafers. Some string ribbon technologies also have higher efficiency levels than other polycrystalline technologies. Overall efficiency on average are from 11-14%. Evergreen is the primary provider of string ribbon solar panels.
Power Conditioning — The process of modifying the characteristics of electrical power (for e.g., inverting direct current to alternating current).
Power Conversion Efficiency — The ratio of output power to input power of the inverter.
Pulse-Width-Modulated (PWM) Wave Inverter — A type of power inverter that produce a high quality voltage.
Pyranometer — An instrument used for measuring global solar irradiance.
Pyrheliometer — An instrument used for measuring direct beam solar irradiance.

Rated Battery Capacity — The term used by battery manufacturers to indicate the maximum amount of energy that can be withdrawn from a battery under specified discharge rate and temperature. See battery capacity.
Rated Module Current (A) — The current output of a photovoltaic module measured at standard test conditions of 1,000 w/m2 and 25C cell temperature.
Rated Power — Rated power of the inverter. However, some units can not produce rated power continuously. See duty rating.
Regulator — Prevents overcharging of batteries by controlling charge cycle-usually adjustable to conform to specific battery needs.
Resistance (R) — The property of a conductor, which opposes the flow of an electric current resulting in the generation of heat in the conducting material. The measure of the resistance of a given conductor is the electromotive force needed for a unit current flow. The unit of resistance is ohms.
Reverse Current Protection — Any method of preventing unwanted current flow from the battery back to the photovoltaic array (usually at night).

Sacrificial Anode — A piece of metal buried near a structure that is to be protected from corrosion. The metal of the sacrificial anode is intended to corrode and reduce the corrosion of the protected structure.
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.
Sealed Battery — A battery with a captive electrolyte and a resealing vent cap, also called a valve-regulated battery. Electrolyte cannot be added.
Seasonal Depth of Discharge — An adjustment factor used in some system sizing procedures which "allows" the battery to be gradually discharged over a 30-90 day period of poor solar insolation. This factor results in a slightly smaller photovoltaic array.
Secondary Battery — A battery that can be recharged.
Self-Discharge — The rate at which a battery, without a load, will lose its charge.
Semiconductor — Any material that has a limited capacity for conducting an electric current.
Series Circuit — Components connected in series are connected along a single path, so the same current flows through all of the components.  In a series circuit, the current through each of the components is the same, and the voltage across the components is the sum of the voltages across all the components.
Series Controller — A charge controller that interrupts the charging current by open-circuiting the photovoltaic (PV) array. The control element is in series with the PV array and battery.
Series Regulator — Type of battery charge regulator where the charging current is controlled by a switch connected in series with the photovoltaic module or array.
Series Resistance — Parasitic resistance to current flow in a cell due to mechanisms such as resistance from the bulk of the semiconductor material, metallic contacts, and interconnections.
Shallow-Cycle Battery — A battery with small plates that cannot withstand many discharges to a low state-of-charge.
Shelf Life of Batteries — The length of time, under specified conditions, that a battery can be stored so that it keeps its guaranteed capacity.
Short-Circuit Current (Isc) — The current flowing freely through an external circuit that has no load or resistance; the maximum current possible.
Shunt Controller — A charge controller that redirects or shunts the charging current away from the battery. The controller requires a large heat sink to dissipate the current from the short-circuited photovoltaic array. Most shunt controllers are for smaller systems producing 30 amperes or less.
Silicon (Si) — A semi-metallic chemical element that makes an excellent semiconductor material for photovoltaic devices. It's commonly found in sand and quartz.
Sine Wave Inverter — An inverter that produces utility-quality, sine wave power forms.
Single-Stage Controller — A charge controller that redirects all charging current as the battery nears full state-of-charge.
Solar Cell — A photovoltaic cell.
Solar Constant — The average amount of solar radiation that reaches the earth's upper atmosphere on a surface perpendicular to the sun's rays; equal to 1353 Watts per square meter or 492 Btu per square foot.
Solar Cooling — The use of solar thermal energy or solar electricity to power a cooling appliance. Photovoltaic systems can power evaporative coolers ("swamp" coolers), heat-pumps, and air conditioners.
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 — A panel consisting of multiple solar cells.
Solar Resource — The amount of solar insolation a site receives, usually measured in kWh/m2/day, which is equivalent to the number of peak sun hours.
Solar Spectrum — The total distribution of electromagnetic radiation emanating from the sun. The different regions of the solar spectrum are described by their wavelength range. The visible region extends from about 390 to 780 nanometers (a nanometer is one billionth of one meter). About 99 percent of solar radiation is contained in a wavelength region from 300 nm (ultraviolet) to 3,000 nm (near-infrared). The combined radiation in the wavelength region from 280 nm to 4,000 nm is called the broadband, or total, solar radiation.
Solar Thermal Electric Systems — Solar energy conversion technologies that convert solar energy to electricity, by heating a working fluid to power a turbine that drives a generator. Examples of these systems include central receiver systems, parabolic dish, and solar trough.
Split-Spectrum Cell — A compound photovoltaic device in which sunlight is first divided into spectral regions by optical means. Each region is then directed to a different photovoltaic cell optimized for converting that portion of the spectrum into electricity. Such a device achieves significantly greater overall conversion of incident sunlight into electricity.
Stand-Alone System — An autonomous or hybrid photovoltaic system not connected to a grid. May or may not have storage, but most stand-alone systems require batteries or some other form of storage.
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.
Standby Current — This is the amount of current (power) used by the inverter when no load is active (lost power). The efficiency of the inverter is lowest when the load demand is low.
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.
String — A number of photovoltaic modules or panels interconnected electrically in series to produce the operating voltage required by the load.
Superconductivity — The abrupt and large increase in electrical conductivity exhibited by some metals as the temperature approaches absolute zero.
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 Operating Voltage — The photovoltaic array output voltage under load. The system operating voltage is dependent on the load or batteries connected to the output terminals.

Temperature Compensation — A circuit that adjusts the charge controller activation points depending on battery temperature. This feature is recommended if the battery temperature is expected to vary more than ±5°C from ambient temperature.
Temperature Factors — It is common for three elements in photovoltaic system sizing to have distinct temperature corrections: a factor used to decrease battery capacity at cold temperatures; a factor used to decrease PV module voltage at high temperatures; and a factor used to decrease the current carrying capability of wire at high temperatures.
Thermophotovoltaic Cell (TPV) — A device where sunlight concentrated onto a absorber heats it to a high temperature, and the thermal radiation emitted by the absorber is used as the energy source for a photovoltaic cell that is designed to maximize conversion efficiency at the wavelength of the thermal radiation.
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.
Total AC Load Demand — The sum of the alternating current loads. This value is important when selecting an inverter.
Total Internal Reflection — The trapping of light by refraction and reflection at critical angles inside a semiconductor device so that it cannot escape the device and must be eventually absorbed by the semiconductor.
Tracking Array — A photovoltaic (PV) array that follows the path of the sun to maximize the solar radiation incident on 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.
Transformer — An electromagnetic device that changes the voltage of alternating current electricity.
Trickle Charge — A charge at a low rate, balancing through self-discharge losses, to maintain a cell or battery in a fully charged condition.
Two-Axis Tracking — A photovoltaic array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal).

Ultraviolet — Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.
Uninterruptible Power Supply (UPS) — The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.
Utility-Interactive Inverter — 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 photovoltaic system's output is fully synchronized with the utility power.


Varistor — A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.
Vented Cell — A battery designed with a vent mechanism to expel gases generated during charging.
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.
Voltage Protection — Many inverters have sensing circuits that will disconnect the unit from the battery if input voltage limits are exceeded.
Voltage Regulation — This indicates the variability in the output voltage. Some loads will not tolerate voltage variations greater than a few percent.

Wafer — A thin sheet of semiconductor (photovoltaic material) made by cutting it from a single crystal or ingot.
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).
Waveform — The shape of the phase power at a certain frequency and amplitude.
Wet Shelf Life — The period of time that a charged battery, when filled with electrolyte, can remain unused before dropping below a specified level of performance.

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