Glossary of Solar Terms

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 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.

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.

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.

FRICTION LOSS
Lost energy 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.

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.

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.

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.

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.

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 (?)
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.

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.

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.

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 per square inch) of water pressure.

PRIMARY CELL
A primary cell is an electrochemical cell (battery) that cannot be recharged. The chemical process within the primary cell is only one way—discharge. When a primary cell is discharged it is discarded. Common flashlight batteries are primary cells; they are disposable batteries that should be avoided.

PSI
See PRESSURE.

PULSE WIDTH MODULATION (PWM)
Varying the amount of DC energy sent to a load or other device by changing the length of time a pulse is left on compared to when it is off. The wider the pulse, the greater the energy transfer.

PURPA
The Public Utility Regulatory Policies Act, passed in 1978. Requires utilities to purchase excess generation from small-scale generators. However, without net metering, this can amount to a fraction of retail prices.

PV
See PHOTOVOLTAIC

PV ARRAY
Two or more photovoltaic modules wired in series or parallel.

PV ARRAY-DIRECT
The use of electricity directly from a photovoltaic array, without batteries or other electrical storage. Many solar water pumps work this way, using a tank to store water.

RADIOTELEPHONE
A two-way radio system that enables use of a regular telephone but with radio instead of wires.

RATED CAPACITY
The manufacturer's specification for the amount of charge that may be stored in a battery, commonly expressed in amp-hours at a specific rate of discharge.

RATED WATT
The manufacturer’s specification for power output of a generating device. In most cases, this is not the most accurate measure to look at, since it predicts output only for ideal circumstances.

RATE OF CHARGE
The amount of energy per unit time that is being added to the battery. Rate of charge is commonly expressed as a ratio of the battery or cell’s rated capacity to charge duration in hours. Example: A C/20 rate on a 100 AH battery would be 5 amps, the capacity of the battery divided by 20.

REGULATOR
A device that prevents overcharging of batteries by controlling the charge cycle, and usually adjustable to conform to specific battery needs. Regulators do not step the voltage down, but control the rate of charge so the battery stays at a specified voltage. Also called CHARGE CONTROLLER.

RENEWABLE ENERGY
Flows of energy that are regenerative or virtually inexhaustible from natural ecological cycles. Most commonly includes solar (electric and thermal), biomass, geothermal, wind, tidal, wave, and hydro power sources.

RESISTANCE
Refers to how well a material conducts a flow of electrons, measured in ohms (?). Resistance is the property of materials to impede a flow of electrons through the material. All materials have some resistance. Those of low resistance are known as conductors, while those of high resistance are known as insulators. The unit used to measure resistance is the Ohm (?).

RESISTOR
A device with a known amount of resistance used in electrical circuits.

REST VOLTAGE
The voltage of a fully charged cell or battery that is neither being charged or discharged.

RF (radio frequency)
Any radiation of a frequency that may be received or radiated by radios. Common usage: RF interference (RFI); refers to the interference of radio frequency radiation with the operation of devices or appliances such as radios, televisions, computers, etc.

RMS
Root mean square; defines a time averaged value of a varying sinusoidal parameter, such as AC voltage, amperage, or wattage. The square root of the average of the squares of a set of numbers.

ROOF MOUNT
A PV or solar collector rack intended to be installed on a roof. For PVs, its elevation angle can be fixed or seasonally adjustable.

ROTOR
The blades of a wind generator, shaped to spin when exposed to wind, harnessing the wind's energy.

RUNNER
The part of a hydro turbine that accepts the water and turns its energy into rotating motion.

R-VALUE
The measure of a material's resistance to heat flow. The higher the R-value, the greater its insulating capabilities.

QUARTZ-HALOGEN LIGHT
An incandescent lamp filled with halogen gas. Somewhat more efficient than standard incandescents.

SECONDARY CELL
Secondary cells are batteries (electrochemical cells) that are rechargeable. The chemical reaction within the secondary cell is reversible, allowing the cell to be recharged many times.

SELF-DISCHARGE
The tendency of all batteries to lose energy. Self-discharge represents energy lost to internal chemical reactions within the cell. This energy is not and cannot be used from the battery or cell’s output terminals.

SENSOR (Temperature)
Sensing device that changes its electrical resistance according to temperature. Used in the control system of a solar thermal system to measure collector and storage tank temperatures.

SERIES CONNECTION
A wiring configuration used to increase voltage from more than one supply. Series wiring is positive to negative (+ to -) or negative to positive (- to +). Opposite of parallel connection. Series circuits have only one possible path for electron flow.

SERIES REGULATOR
A device that prevents overcharging of a battery by disconnecting the charging source as the battery voltage approaches some upper limit.

SERIES STRING
A group of PV modules or batteries wired in series.

SHORT CIRCUIT
A circuit in which two source leads of opposite polarity or dissimilar potential are connected directly to each other with no regulation or load in between, allowing the full energy potential of the source to flow through the circuit. A short circuit will trip the breaker or fuse, and may damage components, or even cause a fire.

SHORT CIRCUIT CURRENT (ISC)
The current between two points in a circuit when the points are electrically connected with a conductor with essentially zero resistance. Normally applied to PV modules, which can be short circuited safely because they are limited current devices.

SHUNT (noun)
1. A resistive load through which electron flow is diverted, typically used to heat air or water.

2. A component with a precise, known resistance used to determine amperage by measuring the voltage across it and using Ohm's law (I = V/R).

SHUNT (verb)
To divert electrical current to a separate circuit or load.

SHUNT REGULATOR
A device that prevents overcharging of a battery by diverting some (or all) of the charging current to a resistive load when the battery voltage reaches a preset upper limit.

SIDE-OF-POLE MOUNT
A PV mount installed on the side of a pole. May be fixed or seasonally adjustable.

SILICON
A nonmetallic element, which when specially treated, is sensitive to light and capable of transforming light into electricity. Silicon is the basic material of most beach sand, and is the raw material used to manufacture most photovoltaic cells.

SINGLE CRYSTAL CELL
A wafer of silicon that has a perfect, continuous, crystal lattice (on the atomic level).

SITE EVALUATION
An estimation of a location for its potential for solar, hydro, or wind power.

SOLAR THERMAL COLLECTORS
A solar collector is a device designed to absorb incident solar radiation and to transfer the energy to the fluid or air passing through it.

SOLAR COOKER
A device that converts the sun's energy into heat energy, which is then used to cook food.

SOLAR-ELECTRIC CELL
See PHOTOVOLTAIC CELL

SOLAR-ELECTRIC MODULE
See PHOTOVOLTAIC MODULE

SOLAR ENERGY
Energy coming directly from the Sun.

SOLSTICE (SUMMER & WINTER
The longest and shortest days of the year. The longest day (Summer Solstice) is about June 21st in the Northern Hemisphere. The shortest day (Winter Solstice) is about December 21st in the Northern Hemisphere.

STAND-ALONE SYSTEM
A system that operates independently of the utility lines. It may draw supplementary electricity from the utility, but is not capable of providing electricity to the utility.

STANDARD TEST CONDITIONS (STC)
The standardized conditions of 1,000 watts per meter squared of solar insolation at 25°C (77°F) for testing PV module ratings.

STATE OF CHARGE (SOC)
A ratio, expressed in percent, of the energy remaining in a battery in relation to its capacity when fully charged.

STORAGE BATTERY
See BATTERY and SECONDARY CELL.

STRATIFICATION
The movement of heat by convection in gasses and liquids causes heat to stratify in layers, the warmest being on top. Stratification is caused by gravity, since the warmer gases and liquids are less dense than the cooler layers.

STRAW BALE CONSTRUCTION
A building technique using straw bales for the walls. See POST AND BEAM CONSTRUCTION.

STRUCTURAL INSULATED PANELS (SIPs)
A no-cavity solid building system of wall and roof panels "sandwiching" polystyrene insulation between an outer and inner sheathing panel (typically oriented strand board (OSB) or metal).

SUSTAINABLE
A material or energy source, which if managed carefully, will provide at current levels indefinitely.

SULFATION
The formation of lead-sulfate crystals on the plates of a lead-acid battery, which decreases battery capacity by impeding the opportunity for chemical reaction within a cell. Sulfation can be caused by leaving the battery in a discharged state for long periods of time.

SURGE CAPACITY
The maximum amount of AC power an inverter may deliver to a load (or loads) for a short period of time, such as when starting a motor.

SWEPT AREA
The area (in square feet or meters^2) that a wind generator’s rotor (blades) sweep. This is the collector area for a wind generator. The larger the collector, the more energy it will capture.

SWITCH
A device that breaks an electrical circuit, halting the flow of electrons through the circuit.

TAIL
The part of a wind generator that makes the rotor face into the wind. Often the tail is also involved in governing the machine, by folding down or sideways to swing the rotor out of the wind.

TAILRACE
The pipe, flume, or channel in a hydroelectric system that carries the water from the turbine runner back to the stream or river.

THERMAL BREAK
A material of low thermal conductivity placed in such a way as to reduce the flow of heat between two materials of high thermal conductivity.

THERMAL MASS
A material that has the ability to absorb, store, and release heat energy. The more heat energy that is required to change the temperature of high-density materials (concrete, bricks, tiles), the more thermal mass the materials have.

THERMOSYPHON
Passive solar hot water systems that rely on the natural convection of liquids to collect energy. Designed with the tank above the collection surface.

THIN FILM
A PV manufacturing technique where silicon is vapor deposited, a few molecules thick, onto another material.

TILT ANGLE
A fixed angle measured from the horizontal to which a solar array is tilted. The tilt angle is chosen to maximize the array output. Depending upon latitude, season, and time of day, the optimum angle will vary.

TILT-UP TOWER
A nonclimbable wind generator tower that tilts up and down to allow installation and servicing of the turbine on the ground. Normally these employ a gin pole—a horizontal lever arm that helps raise and lower the tower.

TOP-OF-POLE MOUNT
See POLE MOUNT.

TRACKER
A mounting rack for a PV array that automatically tilts to follow the daily path of the sun through the sky. A “tracking array” will produce more energy through the course of the day than a “fixed array” (nontracking), particularly during the long days of summer. Some trackers are single-axis while others are dual-axis.

TRANSFORMER
An electrical device that steps up voltage and steps down current proportionally (or vice-versa). Transformers work with AC only.

TRASH RACK
A large strainer at the input to a hydro system. Used to remove debris from the water before it enters the pipe.

TURBINE
An engine that produces rotary motion through reaction or impulse, or both, with moving fluid or gas. The resultant rotary motion is usually used to drive an alternator generator.

TURGO
In hydroelectric systems, a type of impact hydro runner optimized for lower heads and higher volumes than a Pelton runner.

UNINTERRUPTIBLE POWER SUPPLY (UPS)
A power supply providing continuous, uninterruptible service—commonly used in telecommunications and computer networks.

UPWIND
In relation to a wind turbine, toward the wind. An upwind turbine has its blades on the upwind side of the tower.

UTILITY GRID
Commercial electrical energy distribution system. Synonyms: Mains, Grid.

UTILITY-INTERTIE (UI) SYSTEM
See GRID-TIE SYSTEM.

VOLT (V)
The volt is the unit used in the measurement of electromotive force (electrical “pressure”). A standard electrical definition of the volt is: an electromotive force of 1 volt is necessary to move a current of 1 ampere through a 1 ? resistor. It is often also referred to as electrical potential difference or potential difference.

VOLTAGE
A measure of the force or "push" given the electrons in an electrical circuit; a measure of electrical potential. Analogy: pressure in a water pipe. AKA Potential.

VOLTAGE DROP
Loss of voltage (electrical pressure) caused by the resistance in wire and electrical devices. Proper wire sizing will minimize voltage drop, particularly over long distances. Voltage drop is determined by four factors: wire size, current (amps), voltage, and length of wire. Water analogy: friction loss in pipe.

VOLTAGE, NOMINAL
A way of naming a range of voltage to a standard. Example: A “12 volt nominal” system may operate in the range of 10 to 20 Volts. We call it “12 volts” for simplicity.

VOLTAGE, OPEN CIRCUIT
See OPEN CIRCUIT VOLTAGE

VOLTAGE, PEAK POWER POINT (Vpp)
The voltage at which a photovoltaic module or array generates at the highest power (watts). A “12 volt nominal” PV module will typically have a peak power voltage of around 17 volts. A PV array-direct solar pump should reach this voltage in full sun conditions. In a higher voltage array, it will be a multiple of this voltage.

VOLTMETER
A device for measuring the voltage difference between any two points in an electrical circuit.

WATT
Unit of power. Power is the rate of generating or using energy. One watt is the power developed or dissipated in a one volt circuit in which there is a current of one ampere (6.28 million million electrons per second). Watts = amps X volts.

WATT-HOUR
A unit of measurement quantifying an amount of energy used or generated. A load that consumes 1 watt for 10 hours uses 10 watt-hours.