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Technical Glossary

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ABSORBER
The absorber is that part of a solar thermal collector that receives the incident radiant energy and transforms it into heat energy.

ACTIVE SOLAR THERMAL SYSTEM
A system that traps the sun's energy with solar collectors and uses an electromechanical subsystem to move that energy to its point of intended use for water heating, space heating, pool heating, industrial process heat, electrical generation and space cooling.

ALTERNATING CURRENT (AC)
An electrical current in which the direction of electron flow reverses periodically, usually many times per second. Most U.S. household electrical systems use AC current rated at 120 volts and 60 cycles per second.

ALTERNATOR
An mechanical device that generates alternating current electricity.

ALTITUDE ANGLE
The angle of the sun above the horizon, measured in degrees. In winter, the sun is at a low solar altitude, and in the summer, the sun is at a high solar altitude.

AMMETER
A device used for measuring the current (amperage) at any point in an electrical circuit.

AMORPHOUS SILICON
A thin-film PV silicon cell having no crystalline structure.

AMPACITY
Refers to the highest safe amount of electrical current through conductors, overcurrent devices, or other electrical equipment. Ampacity is determined by the cross-sectional area and the material of the conductor, or the manufacturer’s equipment rating.

AMPERE (AMP; A, I)
The rate of flow of electrical charge. Unit of electrical current. One volt across one ohm of resistance causes a current of one ampere. One ampere is equal to 6.235 x 10^18 electrons (one coulomb) per second passing a given point in a circuit.

AMPERE-HOUR (AMP-HOUR; AH)
A measure of electron flow over time, used to measure battery capacity and state of charge. For example, a current of 1 amp drawn from a battery for 10 hours would result in 10 amp-hours of charge cycling through the battery.

AMPERE-HOUR METER
An instrument that monitors electron flow over time. Amp-hours are the product of electron flow (in amperes) and time (in hours).

AMPLITUDE
Generally refers to the maximum and minimum voltage attained by an alternating or pulsed current in each complete cycle or pulse of that current.

ANGLE OF INCIDENCE
The angle between the sun’s rays and a line perpendicular to the active surface of a solar module or collector, in degrees.

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ANGLE OF INCLINATION
The angle that a solar collector or PV module is positioned above horizontal.

ANODE
( Battery ) The electrode within a battery cell that undergoes the chemical process of oxidation. Electrically, the anode is the cell's positive terminal.

(Water heater) An aluminum or magnesium sacrificial rod installed within steel tanks that is used to help prevent corrosion of the tank itself.

ARRAY
Any number of photovoltaic modules connected together electrically to provide a single electrical output.

AWG
American Wire Gauge, a set of standards in the U.S. specifying the diameter of wire. A higher number indicates smaller wire.

AZIMUTH
The angle between true south and a point on the horizon, measured in degrees east or west of true south.

BALANCE OF SYSTEMS (BOS)
Parts or components of a photovoltaic system other than the photovoltaic array or other generating equipment.

BALLAST
A circuit used to condition and stabilize an electric current, for example, in a fluorescent light.

BATCH SOLAR HOT WATER HEATER
The simplest of solar hot water systems. A tank of water within a glass-covered insulated enclosure aimed at the sun. Water is heated in the tank and then flows to the load or an auxiliary water heater.

BATTERY
Two or more electrochemical cells electrically interconnected in an appropriate series/parallel arrangement to provide the required operating voltage and capacity levels. Under common usage, the term battery also applies to a single cell if it constitutes the entire electrochemical storage system.

BATTERY 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, state of charge, age, and cutoff voltage.

BATTERY CELL
The simplest operating unit in a storage battery. It consists of one or more positive electrodes or plates, electrolyte that permits ionic conduction, one or more negative electrodes or plates, separators between plates of opposite polarity, a container for all the above, and posts or other terminals for electrical connection.

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. With lead-acid batteries, end-of-life is generally considered when a fully charged cell can deliver only 80 percent of its rated capacity. Beyond this state of aging, deterioration and loss of capacity begins to accelerate rapidly. Life may be measured in cycles or years, depending on the type of service for which the cell or battery is intended.

BETZ LIMIT
The theoretical maximum energy that a wind generator can extract from the wind—59.6 percent.

BIOMASS
Any organic matter available on a renewable basis, including agricultural crops, wastes, and residues; wood, wood wastes, and residues; animal wastes and municipal wastes; and aquatic plants.

BIOFUELS (BIOMASS FUELS)
Biomass converted directly to energy or converted to liquid or gaseous fuels, such as ethanol methane, and hydrogen.

BLADE
The energy-capturing, aerodynamically designed part of a wind turbine, which interacts directly with the wind.

BLOCKING DIODE
A semiconductor connected in series with a solar-electric cell or cells and a storage battery to keep the battery from discharging through the cell when there is no output, or low output, from the solar cell. It can be thought of as a one-way valve that allows electrons to flow forwards, but not backwards.

BRAKE
Device for stopping a wind turbine. This can be an electric brake that shorts the output of the turbine (dynamic braking), or a mechanical brake that physically stops the rotation, as with a brake drum and shoe.

BREAKER
A manually operable switching device that also automatically opens a circuit in the event of overcurrent.

BRITISH THERMAL UNIT (BTU)
The amount of heat required to raise the temperature of one pound (one pint) of water, one degree Fahrenheit. 1 watt-hour = 3.413 BTU.

BULK CHARGE
The initial phase of battery charging, when the largest amount of energy is put into the battery.

BUSS
An electrical connection component that can accept multiple cables or wires. Also bus, bus bar, or busbar

BYPASS DIODE
A semiconductor device connected in parallel with a series block of parallel PV strings to prevent current from flowing back through any shaded or failed modules in the same block.

CAPACITANCE
An electrical effect in AC circuits that results in amperage peaking before voltage.

CATHODE
The electrode within a battery cell that undergoes the chemical process of reduction. Electrically, the cathode is the negative terminal of the cell.

CATHODIC PROTECTION
Systems that protect metal from corrosion by running small electrical currents along the metal. Most often used to protect well heads, and oil, gas, and water pipelines.

CELL (battery)
A single unit of an electro-chemical device capable of producing an electrical current by converting chemical energy into electrical energy. The cell is the basic unit used to store energy in the battery. The cell contains an anode, a cathode, and the electrolyte. A battery usually consists of several cells electrically connected together to produce higher voltages. (Sometimes the terms cell and battery are used interchangeably).

CELL (solar)
The smallest, basic photovoltaic device that generates electricity when exposed to light.

CHARGE CONTROLLER
A component of photovoltaic systems that controls the charging of the battery to protect the batteries from overcharge and overdischarge. The charge controller may also indicate the system operational status. Standard charge controllers vary the current (A) based on preset voltage set points.

CHARGE RATE
The current applied to a cell or battery to restore its available capacity, specified in relation to total battery size. A C/20 rate is a charge rate that is 1/20th of the total battery capacity. Also called a “20-hour rate.”

CIRCUIT
A group of electrical components that make a complete electrical path, providing some function.

CIRCUIT BREAKER
See BREAKER.

CLOSED LOOP SYSTEM
A solar hot water system of which no part is vented to the atmosphere or fed with fresh liquid. The system liquid, usually some form of antifreeze solution, is recirculated. Closed loop solar systems are also known as glycol systems and indirect systems.

COB CONSTRUCTION
A traditional building technique using hand formed lumps of earth mixed with sand and straw.

COLLECTOR LOOP
The plumbing loop in a solar hot water system that includes the solar collectors. The collectors heat the fluid in the collector, and the heated fluid can be used directly (if water) or the heat can be exchanged to a potable water loop.

COMBINER BOX
A box where wires from individual PV modules or strings are combined into larger wires to run to the battery bank. Can also contain overcurrent protection devices.

COMPACT FLUORESCENT LIGHT (CFL)
A smaller version of standard fluorescent lamps that can directly replace incandescent lights. CFLs use 65 to 80 percent less energy, while producing the same lumens.

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CONCENTRATOR
A photovoltaic module that includes optical components, such as lenses, to direct and concentrate sunlight onto a solar cell of smaller area. Most concentrator arrays must directly face or track the sun.

CONDUCTION
Heat transfer from a hot object to a colder object through direct contact.

CONDUCTOR
A material with relatively low resistance through which electricity will readily flow—wires, cables, busbars. The most common conductors are copper and aluminum.

CONDUIT
Metal or plastic tubing designed to protect electrical conductors.

CONTINUOUS OUTPUT RATING
The maximum amount of power an inverter may deliver to a load (or loads) for a sustained period of time.

CONVECTION

·  Heat transfer by the movement of fluid (usually air or water).

·  Heat transfer through either the natural or forced movement of air.

CONVERTER
An electronic device for DC power that steps up voltage and steps down current proportionally (or vice-versa).

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

CURRENT
Flow rate of electrons. See AMPERE.

CUTOFF VOLTAGE
Electrical equipment setting for the voltage level at which a battery is considered to be empty, and the discharge process is terminated. Most commonly found in inverters and charge controllers that include a feature for low voltage disconnection.

CYCLE

·  One complete charge/discharge cycle of a battery.

·  An AC sine wave’s movement from zero to maximum positive, through zero, to maximum negative, and back to zero.

CYCLE LIFE
Cycle life is the number of cycles a cell or battery will undergo before being considered “worn out.” This point is usually defined as when the battery’s capacity has decreased to 80 percent of its initial rated capacity.

DAYLIGHTING
The placement of windows and skylights to provide natural interior lighting and to reduce daytime electrical demand.

DAYS OF AUTONOMY
The number of consecutive days that a stand-alone renewable energy system will meet a defined load without additional energy input.

DC
Direct current. A one-way flow of electrons. Typical sources of direct current are solar-electric cells, rectifiers, and direct current generators. To be used for typical 120 volt or 220 volt household appliances, DC must be converted to AC (alternating current).

DC MOTOR, BRUSHLESS
High-technology motor used in centrifugal-type DC submersible pumps and other applications. The motor is filled with oil to keep water out. An electronic system is used to precisely alternate the current, causing the motor to spin.

DC MOTOR, BRUSH-TYPE
The traditional DC motor, in which small carbon blocks called “brushes” conduct current into the spinning portion of the motor. They are used in many applications, including DC surface pumps and also in DC submersible diaphragm pumps. Brushes naturally wear down after years of use, and may be replaced.

DC MOTOR, PERMANENT MAGNET
A variable speed motor that uses permanent magnets instead of wound coils. Reduced voltage (in low sun) produces proportionally reduced speed, and causes no harm to the motor.

DEEP-CYCLE BATTERY
A battery designed to regularly discharge 50 to 80 percent of its capacity before recharging.

DEGREE DAY
A quantitative index reflecting demand for energy to heat or cool buildings. Heating and cooling degree days show the difference between the mean daily temperature and a 65°F base. The higher the heating degree days at any location, the colder the winter. The higher the cooling degree days at any location, the hotter the summer.

DELTA T
Difference in temperature.

DEPTH OF DISCHARGE (DOD)
The ampere-hours removed from a fully charged cell or battery, expressed as a percentage of rated capacity. For example, the removal of 25 ampere-hours from a fully charged 100 ampere-hour rated cell results in a 25 percent depth of discharge. Depth of discharge is the opposite of state of charge (SOC).

DHW
Domestic hot water: refers to any system that provides hot water for household use.

DIFFERENTIAL CONTROLLER
An electronic switch that turns off or on based on the difference between two temperatures. In a solar hot water system, the controller measures the temperature at the collector and compares it to the water temperature in a storage tank to turn the pump on or off.

DIGITAL MULTIMETER (DMM)
A device with multiple electrical measurement capabilities, such as voltage, amperage, resistance, etc., commonly usable for both AC and DC circuits. It has a digital display.

DIODE
A semiconductor device that allows electrical current in only one direction.

DIRECT CURRENT (DC)
An electrical current that moves in one direction only.

DIRECT GAIN
In passive solar heating, a direct gain system relies on the sunshine to directly hit the substance or mass being heated. Direct gain systems used today usually rely on a layer(s) of glass to assist in holding the heat within a space where the heat is desirable.

DISCHARGE RATE
The rate at which energy is being drained from a battery.

DISCONNECT
Switch gear used to connect or disconnect components in a system.

DOWNWIND
In relation to a wind turbine, the direction away from the source of wind. A downwind turbine has its blades on the downwind side of the tower.

DRAFT TUBE
A tube added to the outfall of a hydro turbine to increase energy production by taking advantage of the drop in the tailrace.

DRAINBACK SYSTEM
A solar hot water system that only fills the collector when the temperature differential is appropriate. The water that is circulated through the collectors is stored in a reservoir. Draining the collectors provides freeze protection.

DRAINDOWN SYSTEM
A solar hot water system that uses a special draindown valve that redirects the collector fluid and drains it down when the collector system pump is not operating. These systems have been prone to failure and are not recommended.

DUTY CYCLE
The fraction of time a device or load actually uses energy in a unit of time. For example, a load that uses energy for 5 seconds out of every 10 seconds has a 50 percent duty cycle.

EARTH
Synonymous with “ground.”

EFFICIENCY (PV modules)
The ratio of power output of a photovoltaic cell to the incident power from the sun or simulated sun sources under specified standard insolation conditions. A solar cell that converts 1/10 of the sun's energy that strikes its surface to electricity has an efficiency of 10 percent.

EFFICIENCY
The effectiveness of a device to convert energy from one form to another, or to transfer energy from one body to another. An electric pump that is 60 percent efficient converts 60 percent of the input energy into work—pumping water. The remaining 40 percent becomes waste heat.

ELECTRICAL POTENTIAL
Same as VOLTAGE.

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ELECTRIC CURRENT
The rate at which electrons flow through an electrical conductor, usually measured in amperes (amps).

ELECTRICITY
Energy flow resulting from the flow of charged particles, such as electrons or ions.

ELECTROLYSIS
The breaking down of a chemical compound into simpler compounds or elements by the passage of electricity through the chemical compound. Commonly used to describe the extraction of hydrogen (and oxygen) from water.

ELECTROLYTE
The fluid in a battery, which is the medium of ion transport within an electrochemical cell. It provides a path for electron transfer between the anode and cathode of a battery cell.

ELECTROMAGNETIC RADIATION (EMR)
Magnetic radiation produced by a changing electrical current, such as alternating current (AC).

ELECTRON
A negatively charged particle. The movement of electrons in an electrical conductor constitutes an electric current.

EMBODIED ENERGY
The energy consumed by all of the processes associated with the production of a material. This includes the energy required in mining, transport, manufacturing, administration, use, disposal, etc.

ENERGY
The amount of work that a system or entity can do (potential energy) or is doing (kinetic energy), measured in joules. The product of power and time, measured in watt-hours. 1,000 watt-hours = 1 kilowatt-hour (KWH).

ENERGY DENSITY
A ratio of a battery or cell's capacity to either its volume or weight. Volumetric energy density is expressed in watt-hours per cubic inch. Weight energy density is expressed in watt-hours per pound.

ENGINE
A machine that converts energy into mechanical force or motion. Sources of energy include heat, chemical reaction, potential energy of elevated water, etc.

EQUALIZATION
The process of restoring all cells in a battery to an equal state-of-charge. For lead-acid batteries, this is a charging process designed to bring all cells to 100 percent state-of-charge.

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.

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EQUINOX (SPRING & FALL)
The time when the sun crosses the plane of the earth's equator, making night and day of equal length all over the earth, occurring about March 21st and September 21st.

EVAPORATION
The process of a liquid changing its state into a gas when heat is added. In the most common occurrence on earth, water evaporation requires 970 BTUs per pound (pint).

FEATHERING
In wind generators, this refers to an adjustment of the blades so that they catch less wind. This can prevent damage to the machine in high winds.

FINISH CHARGE
The final stage of battery charging, when the battery is charged at a slow rate over a long period of time.

FLAGGING
Noticeable deformation of trees from prevailing winds. Flagging is an indication of an effective wind site. Lack of flagging is not necessarily an indication of a poor wind site.

FLAT PLATE COLLECTOR
A solar thermal collector that converts the sun’s radiation into heat on a flat surface. Does not use reflecting surfaces or lens arrangements to concentrate the sun’s energy.

FLOAT CHARGE
A trickle charge to keep a battery fully charged at a safe voltage level with minimal gassing.

FLOAT SERVICE
A battery operation in which the battery is normally connected to an external current source; for instance, a battery charger that supplies the battery load under normal conditions, while also providing enough energy input to the battery to make up for its internal losses, thus keeping the battery always at full charge and ready for service.

FLOW
In hydro-electric terms, flow refers to the quantity of water supplied to a water source or exiting a nozzle per unit of time. Commonly measured in gallons per minute.

FLUORESCENT LIGHT
An electric lamp coated on its inner surface with phosphor and containing mercury vapor. When bombarded with electrons, the vapor emits ultraviolet light that causes the phosphor to emit visible light.

FRANCIS TURBINE
A type of reaction hydro-turbine used in low to medium heads. It consists of fixed vanes on a shaft. Water flows down through the vanes and out sideways.

FREESTANDING TOWER
A wind generator tower with no guy wires. This can be either a lattice tower or a monopole. Freestanding towers are the most expensive type of tower, requiring large excavations and large amounts of concrete.

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FRICTION LOSS
Lost energy due to friction.

·  In hydro systems, pipe sized too small can lead to serious friction losses.

·  In any belt drive system, there will be some losses due to friction.

FULL SUN
The full sun condition is the amount of power density received at the surface of the earth at noon on a clear day—about 1 KW per m^2, or 1 Sun. Lower levels of sunlight are often expressed as 0.5 sun or 0.1 sun.

FURLING
Reducing a wind generator’s swept area to protect it from high winds. Common furling methods are to tilt the rotor (blades) up or sideways out of the wind, or to feather (twist) the blades to degrade the airfoil.

FUSE
An electrical device that is designed to break a circuit by melting an internal conductor when the current in the circuit exceeds the maximum considered safe for the conductors or devices in the circuit.

GASSING
The production of hydrogen and oxygen gas from one or more of the electrodes in the cells of a battery. Gassing commonly results from the electrolysis of water in the electrolyte during charging.

GENERATOR
A device that converts mechanical energy into electrical energy.

GEOTHERMAL HEAT PUMP
A type of heat pump that uses the ground, ground water, or ponds as a heat source and heat sink, rather than outside air. Compare to HEAT PUMP.

GIN POLE
Either of two different types of devices used with wind generator towers. With a tilt-up tower, it describes the lever that helps tilt the tower up. With a fixed tower, it describes a temporary crane used to raise tower sections or the wind generator.

GLYCOL (Propylene Glycol)
An antifreeze, heat transfer fluid that is circulated through closed loop solar hot water collectors.

GOVERNOR
A device that limits the output of another device, such as a wind generator.

GOVERNING
Limiting the output of a device. In respect to small wind generators, governing normally happens through furling.

GRID
Transmission line network used to distribute electrical energy, generally by a commercial power utility.

GRID LINES
Metallic contacts fused to the surface of a solar cell to provide a low resistance path for electrons to flow out to the cell interconnect wires.

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GRID-TIE SYSTEM
A renewable energy system that is connected to the utility grid, selling excess energy back to the utility. Also called a utility-interactive system.

GROUND
The connection of electrical components to the earth and/or each other for the purposes of dissipating static charge or protecting against a short circuit or lightning.

GROUND FAULT
Unwanted current path to ground.

GROUND MOUNT
A photovoltaic (PV) rack designed to be installed on the ground or other flat surface.

GROUND ROD (ELECTRODE)
A metal rod (typically 5/8 inch diameter) that is driven into the earth (typically 8 feet deep) and is electrically connected to the negative conductor and/or any metal parts, wiring enclosures, or conduit of an electrical circuit.

GUY WIRES
Steel cables that support a tower.

HEAD
The difference in elevation between two parts of a liquid-based system. In hydro power, the difference between a source of water and the location at which the water from that source may be used (synonym: vertical drop). With pumps, the vertical distance the pump must move the water.

HEADRACE
A flume or channel that feeds water into a hydro turbine.

HEAT EXCHANGER
A device that is used to transfer heat between fluids or gases through an intervening surface.

HEAT PUMP
A device typically used for heating and cooling of buildings by drawing from or dissipating into the ambient temperature of air or water. When cooling, a heat pump works like a refrigerator. When heating, it also works like a fridge, except the heat produced is used to heat a space.

HEAT SINK
A medium or container to which heat flows. Thermal mass walls and floors in a passive solar home act as a heat sink during the day.

HEAT TRANSFER
Heat is transferred from one substance or location to another by three methods—radiation, convection and conduction. The sun's rays are a good example of radiation; warm air rising is heat movement by convection; and touching a hot iron or frying pan with your hand is heat transfer by conduction.

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HERTZ (Hz)
Cycles per second. Generally refers to the number of complete cycles of the AC sine wave per second, or the frequency at which a radio or computer processor works.

HYDRO-ELECTRICITY
Any electricity that is generated by the flow of water.

HYDROGEN FUEL CELL
A device that converts hydrogen to DC electricity.

HYDROMETER
A hydrometer is an instrument for measuring the density of liquids in relation to the density of water. The hydrometer is used to indicate the state of charge in lead-acid cells by measuring the specific gravity of the electrolyte.

HYDRONIC HEATING SYSTEM
A type of heating system where water is heated in a solar collector or boiler, and is pumped to heat exchangers or radiators in rooms. Radiant floor systems have a grid of tubing laid out in the floor for distributing the heat. Temperature of the space is controlled by regulating the flow and/or temperature of the circulating water.

HORSEPOWER
A measure of the capacity to generate energy or do work. 1 horsepower = 746 watts.

HYBRID SYSTEM
An energy system consisting of two or more generating subsystems, such as the combination of a wind turbine or diesel generator and a photovoltaic system.

INCANDESCENT LIGHT
An electric lamp that is evacuated or filled with an inert gas and contains a filament (commonly tungsten). The filament emits visible light when heated to extreme temperatures by electric current through it. Incandescent lightbulbs are one of the most inefficient ways to light a home. They produce a great deal of heat along with the light, and use three to four times as much energy for the same light output as compact fluorescent lightbulbs.

INDUCTION MOTOR (AC)
A type of electric motor that requires a high surge to start, and a stable voltage supply, making it a challenge to run using a solar-electric system.

INSOLATION
The amount of sunlight reaching an area. Usually expressed in watts per square meter.

INTAKE
In a hydro system, the structure that receives the water and feeds it into the penstock (pipeline). Usually incorporates screening or filtering to keep debris and aquatic life out of the system.

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INVERTER
A device that converts DC electricity (anywhere from 12 to 600 VDC) to AC electricity (typically 120/240 VAC).

ION
An electrically charged particle or molecule.

ISC
See SHORT CIRCUIT CURRENT.

IV CURVE
The graphical representation of the current versus the voltage of a photovoltaic cell, module, or array as the load is increased from zero voltage to maximum voltage, under standard test conditions.

JACK PUMP
A submerged pump mechanically activated by a rod extending above the well head to a reciprocating engine, motor or any other rotating device.

JOULE
The standard unit of energy (SI). One joule equals one watt-second, and 3600 joules = one watt-hour.

KILOWATT (KW)
One thousand watts.

KILOWATT-HOUR
One thousand watt-hours. Unit of energy used to perform work (energy and work are equivalent in units, energy being the potential value and work the achieved value).

Fuel equivalents: One barrel of crude oil contains roughly 1,700 KWH

One ton of coal contains roughly 7,500 KWH

One gallon of gasoline contains roughly 37 KWH

One cubic foot of natural gas contains 0.3 KWH

One ton of uranium ore contains 164 million KWH

1.34 horsepower-hours.

1 KWH = 3,400 BTU. Can be compared to 860 calories (food energy value).

LATITUDE
A location's distance north or south of the equator measured in degrees.

LIGHT EMITTING DIODE (LED)
A semiconductor device composed of a PN junction designed such that electrons cause visible light during their migration across the junction.

LIGHTNING ARRESTOR
Devices that protect electronics from lightning-induced surges by carrying the charge to ground.

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LINE/WIRE LOSS
The voltage drop or energy loss due to the resistance of wire in an electrical circuit. See VOLTAGE DROP

LINEAR CURRENT BOOSTER (LCB)
An electronic circuit that matches PV output directly to a motor by converting unneeded voltage to higher usable current. Used in array-direct water pumping.

LOAD
Refers to equipment that is powered by electricity. Usually expressed in terms of amperes or watts. Any device or appliance that uses energy (such as a light or pump).

LOW-E GLASS
Glass coated with layers of metal or metal oxide. The coating emits very little radiation in the long-wave (infrared) spectrum, which diminishes heat loss from the building interior, and reduces heat gain in hot weather.

LUMEN
A unit of measurement quantifying the amount of light emitted from a light source.

MAGNETIC DECLINATION
The number of degrees east or west of true south from magnetic south.

MAXIMUM POWER POINT
The point on a PV module’s voltage/amperage curve where the product of current and voltage is a maximum (measured in watts).

MAXIMUM POWER POINT TRACKING (MPPT)
Electronically tracking the maximum power point of a PV module to achieve the highest possible output, by (in simplest terms) using surplus voltage to boost amperage.

MICROHYDRO
Hydro-electric plants producing more than 100 watts and less than 2,000 watts.

MODULE
The smallest nondivisible, self-contained, and environmentally protected physical structure housing interconnected photovoltaic cells and providing a single DC electrical output. Commonly called a “panel.”

MOTOR
A device that converts electrical energy into mechanical energy.

MULTICRYSTALLINE CELL
See POLYCRYSTALLINE CELL

NANOHYDRO
Any hydro plant that produces less than 100 watts.

NATIONAL ELECTRICAL CODE (NEC)
A document that describes the legal standards for residential and commercial wiring practices with safety as the prime objective. Many U.S. jurisdictions base their wiring inspections on the NEC.

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NET METERING
State by state legislation that requires utilities to purchase renewably produced electricity at the same price that they sell it, until a building's monthly or annual energy use is offset.

NOMINAL VOLTAGE
A reference voltage used to describe batteries, modules, or systems (for example, a 12 volt or 24 volt battery, module, or system).

NOSECONE
The pointed piece farthest toward the wind on a wind generator, designed primarily for cosmetic purposes, but also protects the blade attachment points and generator from the weather.

OFF-PEAK
The period of low energy demand, as opposed to maximum or peak demand.

ON-PEAK
Energy supplied during periods of relatively high system demands as specified by the utility.

OHM (Ω)
The unit that quantifies a material's resistance to electron flow.

OHM’S LAW
Basic formula defining the relationship between voltage, amperage, and resistance. Commonly stated as E = I x R, or Voltage = Amperage x Resistance.

OPEN CIRCUIT
When an electrical circuit is interrupted by breaking the path at one or more points, stopping the electrons from flowing. A light switch opens an electrical circuit when it turns off the light.

OPEN CIRCUIT VOLTAGE (VOC)
The maximum possible voltage across a PV array, module, or cell. The voltage across the terminals of a photovoltaic cell, module, or array with no load applied when the cell is exposed to standard insolation conditions, measured with a voltmeter.

OPEN LOOP SYSTEM
A fresh water or "direct" solar hot water system, generally for use in freeze-free climates.

ORIENTATION
Placement according to the directions N, S, E, or W.

OUTGASSING
The emission of gasses by a material. See also GASSING.

OVERCURRENT
Current that exceeds the rated current of the equipment or the ampacity of a conductor, resulting from overload, short circuit, or ground fault.

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OVERCURRENT DEVICE
A safety fuse or breaker designed to open a circuit when an overcurrent occurs.

PARALLEL CONNECTION
An electrical circuit with more than one possible path for electron flow. When wiring PV modules, this wiring configuration increases amperage (current), while voltage remains the same. Parallel wiring is positive to positive (+ to +) and negative to negative (- to -). Opposite of a series connection.

PASSIVE SOLAR
Any use of the sun's energy in a manner that is found in nature without the use of mechanical aid like pumps or fans. For example, heating a thermal mass (a concrete wall or slab, for instance) during the day with direct sunlight, and using the stored heat in that mass to warm a greenhouse or home at night.

PAYBACK
The period of time it takes for an energy generating device or system to pay for itself in fuel savings.

PEAK LOAD
The maximum load or electrical power draw occurring in a given period of time.

PEAK POWER POINT
Operating point of the IV (current-voltage) curve for a photovoltaic cell or module where the product of the current value times the voltage value is a maximum. Also called the “maximum power point.”

PEAK SUN HOURS
The equivalent number of hours per day when solar irradiance averages 1,000 watts per meter squared.

PELTON WHEEL
A common impulse turbine runner—the wheel that receives the water, changing the pressure and flow of the water to circular motion to drive an alternator, generator, or machine. Pelton wheels (named after inventor Lester Pelton) are made with a series of cups or “buckets” cast onto a hub.

PENSTOCK
The pipe in a hydro system that carries the water from the intake to the turbine.

PHANTOM LOAD
A device that consumes energy even when its switch is off, such as the digital clock on a VCR.

PHOTON
The actual (physical) particle unit of light, as the electron is a particle of electric charge. Solar-electric modules use photons to generate electricity. Photons not captured by the cell are either reflected, pass through the panel, or are converted to heat in the solar array.

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PHOTOVOLTAIC ARRAY
A collection of solar modules connected in series, parallel, or series-parallel combination to provide greater voltage, current, or power than can be furnished by a single solar module. Solar-electric arrays can be designed to furnish any desired voltage, current, or power.

PHOTOVOLTAIC CELL
A device composed of specially prepared semiconductor material or material combinations exhibiting the ability to convert incident solar energy directly into electrical energy.

PHOTOVOLTAIC 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, electrons will flow and electrical energy will be available.

PHOTOVOLTAIC MODULE
A PV module consists of series and/or parallel wired cells typically made from layered silicon crystals that convert light energy to DC electricity. The number of modules in a given system varies depending on the combined load being powered.

PIPE LOSS (Frictional head loss)
The amount of energy or pressure lost due to friction between a flowing liquid and the inside surface of a pipe.

P-N JUNCTION
The semiconductor junction in a photovoltaic cell that shunts electrons into a circuit. Electrons are bumped across this junction by photons (light particles).

POLE MOUNT
A PV mount that is installed on the top or side of a pole usually set in concrete. Can be fixed or seasonally tilted.

POLYCRYSTALLINE CELL
A wafer of silicon with a multi-grained structure. All grains have the same atomic crystal lattice, however, each grain has a unique orientation in space, producing a unique reflection of light, resulting in a "patchy" mottled appearance. AKA multicrystalline cell.

POST AND BEAM CONSTRUCTION
A traditional building technique in which post and beam framing units are the basic load-bearing members. Post and beams may be of wood, structural steel, or concrete. In this system, there are fewer framing members, leaving more open space for in-fill. Often used in straw bale construction.

POWER
The rate of energy use or generation per unit time, measured in watts. 1 watt = 1 joule per second.

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POWER FACTOR
The ratio of real power (watts) to apparent power (volt-amps) in an AC circuit. Power factor describes the offset between voltage and amperage peaks in AC. 1 is called “unity” power factor, and is when voltage and amperage peak at the same time—they are then said to be “in phase.” Power factor is calculated by dividing W by VA.

PRESSURE
The “push” behind liquid or gas in a tank, reservoir, or pipe. Water pressure is directly related to “head”—the height of the top of the water over the bottom. Every 2.31 feet of vertical head gives 1 psi (pound