Glossary of Basic Renewable Energy Terminology
(Note: Some terms are specific only to Texas or other states.)

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Updated: January 6, 2017.

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AC-Coupled System

An AC-coupled system combines batteryless and battery-based inverters in the same system. In AC-coupled systems, one or more grid-tied inverters can work with certain battery based inverters to disconnect from the utility grid and continue to supply electricity for critical loads when the utility power fails. Special battery-based inverters are required in ac-coupled systems. See also "Grid-tied Inverter". (Note: Do not attempt to create an ac-coupled system with a conventional battery-based inverter.)

AC Module

An inverter that's permanently attached to and an integral part of a photovoltaic module. All DC wiring is completely internal to an ac-module inverter. For that reason, unlike "micro-inverters", ac module inverters are not required to have DC-side ground-fault detection. Not to be confused with "micro-inverter" (also "microinverter"). See also "micro-inverter".


The fraction of solar radiation reflected from the ground, ground cover, and bodies of water on the surface of the earth.

Alternating Current

Electric current that reverses direction 50 or 60 times per second (depending on the country).

Amorphous Photovoltaic

Amorphous photovoltaic modules are commonly made from a special form of silicon that's applied like other "thin-film" materials. Typically browish in color. Often found used in calculators and similar applications.


Electrical current. A measure of the quantity of electricity flowing in a circuit. (Think of "Amps" as if they were gallons of water, and "Amps" are the gallons flowing per minute through a pipe.)

(Amp-hours, A-H, AH)

Often abbreviated "A-H" or "AH". A measure of the amount (quantity) of electrical current flowing for a given period of time. Typical time unit is "hours", thus "amp-hours".

Aperture Area

At the PV module level, the total area including the frame may be used to define the "module area efficiency". For prototype modules, where the frame design is less important than the encapsulation and cell interconnections, an "aperture-area" definition is often used. The aperture-area is the total PV area minus the frame area. Aperture-area calculations yield a higher efficiency value due to the smaller area compared to the entire PV module. However, in modules where the PV frame represents a notable percentage of the total area, it's the total area efficiency that often matters.


Two or more photovoltaic modules connected together.

Azimuth Angle

The angle between the horizontal direction (of the sun, for example) and a reference direction (usually north). A typical azimuth angle for photovoltaic modules in America would be 180 degrees (facing south at latitudes north of the equator), and 0 degrees (facing north at latitudes south of the equator, such as South America Australia, and others).






A fuel containing a mixture of 20 percent biodiesel and 80 percent petroleum-based diesel.


A fuel containing a mixture of 50 percent biodiesel and 80 percent petroleum-based diesel.


All parts of a solar electric system OTHER than the photovoltaic modules. Balance-of-system components include but are not limited to inverters, racking, wires, conduit, switches, fuses, and so forth.

Base Load

The average minimum amount of electricity a utility company must supply to meet consumer electricity demand.

Baseload Capacity

The generating equipment normally operated to serve loads on an around-the-clock basis.


Biodiesel is an alternative fuel produced from renewable resources such as plant oils, animal fats, used cooking oil, and new sources such as algae. Biodiesel contains no petroleum, but can be combined in any quantity with petroleum diesel to create a biodiesel blend. Biodiesel blends can be used in most "compression-ignition" (diesel) engines with little or no modifcations. Biodiesel is simple to use, biodegradable, nontoxic, and essentially free of sulfur and aromatics (the "diesel" odor). (Biodiesel is not raw vegetable oil.)


Liquid fuels and blending components produced from biomass (plant) feedstocks, used primarily for transportation.


Biomass means "natural material", and refers to biological materials that were alive or created during our lifetimes. When burned, biomass materials release heat - just like wood logs in a campfire. Biomass energy uses natural materials like trees and plants. It can also mean waste products like trash. Examples of biomass include grass clippings, wood chips, animal manure, and non-toxic trash. Biomass exists in landfills, where bacteria break down the waste material, creating methane gas in the process, which can be captured and burned. Energy from biomass is most often captured to generate electricity. Biomass specifically excludes coal and petroleum.

British Thermal Units

Often abbreviated "Btu" or "btu". The amount of heat energy needed to raise the temperature of one pound of water by one degree F (for example, from 39 degrees F to 40 degrees F). A water temperature of 39 degrees F is used as the reference starting point because water is the most dense at this temperature. Air conditioners are rated in three ways: by BTUs; tons of refrigeration, or by horsepower. One ton of refrigeration removes the amount of heat needed to melt one ton of ice in 24 hours. One ton of refrigeration can remove 12,000 BTUs of heat in one hour, and would use approximately 3,517 watt-hours of electrical energy. To convert a "one ton" air conditioning unit into kilowatts-per-hour consumed is dependent type of air conditioner, its Energy Efficiency Rating ("EER"), and the environment in which it's operated. For instance, a typical one-ton air conditioner operating 20 minutes out of each hour consumes approximately 1,335 watt-hours (1.335 kilowatt-hours) of electrical energy.

Building-Integrated Photovoltaics ("BIPV")

Photovoltaic products (see "PV") used in the construction of buildings that replace standard building materials, such as the roof, walkways, windows, and awnings.





California Energy Commission ("CEC")

The California Energy Commission ("CEC")is the State of California's primary energy policy and planning agency. Among their missions is supporting renewable energy by providing market support to existing, new, and emerging renewable technologies; providing incentives for small wind and fuel cell electricity systems; and providing incentives for solar electricity systems in new home construction. The CEC requires independent performance certification on renewable energy products sold within the state of California. Certification tests are conducted under more real-world conditions. Many other states now rely on the product specifications from the CEC published on the CEC "Go Solar California" website.

Capacity Factor

The ratio of the electrical energy produced by a generating unit for a given period of time to the electrical energy that could have been produced at continuous full power operation during the same period.


Abbreviation for "cadmium telluride", a combination of two metals used in the manufacture of photovoltaic modules. See "Thin-Film".


Abbreviation for "copper indium gallium selenide", a type of photovoltaic module manufacturing technology. See "Thin-Film".

Compact Fluorescent Light ("CFL")

Fluorescent light manufactured to occupy a very small area and able to be installed in an ordinary light fixture. CFL bulbs use a fraction of the electricity used by incandescent light bulbs.

Concentrated Solar Power

A solar energy conversion system characterized by the optical concentration of solar rays through an arrangement of mirrors to heat working fluid to a high temperature. Concentrated solar power (but not solar thermal power) may also refer to a system that focuses solar rays on a photovoltaic cell to increase conversion efficiency.


The reduction of energy usage through increased efficiency and/or reduced waste.

Conversion Efficiency

A measure that gauges the percentage of solar energy reaching a module that in turn is converted into electrical power.

Cooling Degree-Days

The amount of air-conditioning needed, created by adding up all temperature differences of the form ( daily temperature in degrees F - 65 degrees F ) for each day in which the temperature exceeds 65F. See also "heating degree days".

Crystalline Modules ("c-Si")

Photovoltaic cells made from thinly-sliced silicon. See also "Monocrystalline" and "Polycrystalline".





Direct Current Electricity ("DC")

Electricity that flows only in one direction. Direct Current is the type of electricity supplied by batteries.


Sometimes "US DOE". In this context, an abbreviation for the United States Department of Energy.






A fuel containing a mixture of 85 percent ethanol (alcohol) and 15 percent gasoline.


EEM is an abbreviation for Energy Efficient Mortgage. When you are buying, selling, refinancing, or remodeling your home, you can increase your comfort and actually save money by using the Energy Efficient Mortgage (EEM). It is easy to use, federally recognized, and can be applied to most home mortgages. EEMs provide the borrower with special benefits when purchasing a home that is energy efficient, or can be made efficient through the installation of energy-saving improvements. To learn more about Energy Efficient Mortgages, click here.


In this context, "Elevation" refers to either the height of the sun in the sky relative to the horizon (measured in degrees), or the tilt angle of photovoltaic modules or other solar energy collectors. Sun elevation is zero degrees when the sun is directly on the horizon (as in the exact moment of sunrise or sunset), to 90 degrees when directly overhead. A sun angle of 90 degrees occurs only at latitudes between the tropic of cancer through the tropic of capricorn. Photovoltaic module elevation is zero degrees when perfectly horizontal relative to the earth's surface, and 90 degrees when perpendicular to the earth's surface.

Energy Efficient Appliances

Electrical devices or appliances that perform their task, and use less electricity than lower-efficient devices. Electrical inefficiency in many devices is directly related to the heat they produce. For example, energy efficient light bulbs use most of the incoming electrical energy to produce light, not heat.


The Electric Power Research Institute, a research consortium of electric power companies in the United States.


The Electric Reliability Council of Texas ("ERCOT") manages the flow of electric power to 23 million Texas customers representing 85 percent of the state's electric load. As the independent system operator for the region, ERCOT schedules power on an electric grid that connects 40,500 miles of transmission lines and more than 550 generating stations.


A clear, colorless, flammable oxygenated hydrocarbon. Ethanol is typically produced chemically from ethylene, or biologically from fermentation of various sugars from carbohydrates found in agricultural crops and cellulosic residues from crops or wood. It is used in the United States as a gasoline octane enhancer and oxygenate (blended up to 10 percent concentration). Ethanol can also be used in high concentrations (E85) in vehicles designed for its use.


"EUI" is an abbreviation for "Energy Use Intensity", and is expressed as energy consumed per square foot per year. It's difficult to compare energy consumption between buildings or structures. Simply measuring the amount of energy used in a given amount of time doesn't consider building size, configuration, or how it's used. For example, a public building such as a school would tend to use more energy per square foot than a home. EUI is calculated by dividing the total gross energy consumed in a one-year period (expressed in kilowatt-hours ["kW-h"]or thousands of British Thermal Units ["BTU"]) by the total gross square footage of the building. A low "EUI" generally indicates good energy performance.


Abbreviation for "Electric Vehicle", a vehicle that derives all of its ability to move from energy stored in batteries, and does not have an internal combustion engine of any type.


"E-waste" is anything electronic that's put into the garbage. E-waste includes (but isn't limited to) old computers, printers, modems, scanners, mobile phones, televisions, stereo equipment, and radios .. for openers. E-waste typically contains lead, cadmium, selenium, phosphors, precious metals, plastics, fiberglass, chromium, steel, aluminum, and various assorted plastics, among many other materials.





Federal Energy Regulatory Commission

The federal agency with jurisdiction over interstate electricity sales, wholesale electric rates, hydroelectric licensing, some natural gas pricing, oil pipeline rates, and gas pipeline certification. FERC is an independent regulatory agency within the Department of Energy (DOE) and is the successor to the Federal Power Commission.


Money paid to a customer by a power company for excess electricity generated by a renewable energy source. The renewable energy source is most often either solar or wind generated electricity. This excess is connected to the power lines at the customer's residence, and most commonly on the customer's side of the electric meter. For example, if you had a solar electric system installed on your home, and had signed a feed-in-tariff agreeement ("FIT") with your power company, you could find the power company selling electricity to you at one price per kilowatt-hour, but BUYING the excess from you for more than they sell it to you at retail. Feed-in-tariffs are implemented to encourage end users to install renewable energy equipment and sell excess renewable energy back to the utility company.


The part of an incandescent lamp through which electricity passes and gets extremely hot, producing light.

Flat-Plate Collector

A solar power collector that absorbs the sun's energy on a flat surface without concentrating or refocusing it.

Flexible-Fuel Vehicles

Vehicles that can operate on (1) alternative fuels (such as E85); (2) 100 percent petroleum-based fuels; (3) any mixture of an alternative fuel (or fuels) and a petroleum-based fuel. Flexible-fuel vehicles have a single fuel system to handle alternative and petroleum-based fuels.

Fluorescent light

A lighting device that uses an electrified gas rather than a filament to produce light.

Fuel Cell

A device that produces electricity by converting the chemical energy of a fuel (e.g., hydrogen) directly into electrical energy. Fuel cells differ from conventional electrical cells in that the active materials such as fuel and oxygen are not contained within the cell but are supplied from outside. It does not contain an intermediate heat cycle, as do most other electrical generation techniques. In this context, "fuel cells" are most commonly a device that combines hydrogen gas with oxygen (usually derived from the air around us rather than a separate storage tank) to produce electricity. Hydrogen-oxygen fuel cells typically convert approximately 40 to 60% of the available "fuel" energy to electricity. (Compare this to the typical 20-25% efficiency of the common automobile engine.) The waste by-products of a hydrogen-oxygen fuel cell are heat, and water (or water vapor).






The total amount of electric energy produced by generating units and measured at the generating terminal in kilowatt-hours (kWh) or megawatt-hours (MWh).


Heat from the earth. Often thought of as energy from geysers and hot springs. More recently, this term is applied to any heat stored in earth and available as a renewable energy resource.

Geothermal Energy

The heat that is extracted from hot water or steam that is mined from geothermal reservoirs in the earth's crust. Water or steam can be used as a working fluid for geothermal heat pumps, water heating, or electricity generation, and then is injected back into the earth.

Geothermal Heat Pump

A heat pump in which the refrigerant exchanges heat (in a heat exchanger) with a fluid circulating through an earth connection medium (ground or ground water). The fluid is contained in a variety of loop (pipe) configurations depending on the temperature of the ground and the ground area available. Loops may be installed horizontally or vertically in the ground or submersed in a body of water.


From "giga", meaning billion, and "watt", a unit of energy (see "watt"). A gigawatt is one billion watts of electrical energy. To give you an idea how large one gigawatt is, it's the energy consumed by 10 million 100 watt light bulbs illuminated at the same time.


One billion watt-hours. (This is the equivalent energy consumed by 1,900 sixty-watt incandescent lights operating 24 hours a day for one year.)

Global Warming

The Earth's gradual warming due to the "greenhouse effect".

Greenhouse Effect

The rise in temperature the Earth experiences because certain gases in the atmosphere (such as water vapor, carbon dioxide, nitrous oxide, and methane) trap energy from the sun. Without these gases, heat would escape back into space and Earth's average temperature would be about 60 degrees F colder. Because of how they warm our world, these gases are referred to as greenhouse gases.

Greenhouse Gases

Gases in the Earth's atmosphere that produce the greenhouse effect. Changes in the concentration of certain greenhouse gases, due to human activity such as fossil fuel burning, increase the risk of global climate change. Greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, ozone, and various forms of fluorocarbon gas (used in air conditioners and refrigerators).


The network of wires and cables that transport electricity from a power plant to homes and business.

Grid-tied Inverter

An inverter that converts direct current (DC) into alternating current (AC) and feeds the power into the utility grid of homes and businesses on the customer's side of the meter. This has the effect of reducing the amount of power purchased from the utility company. In certain cases, more power may be produced by the system, in which case the utility meter may run "backwards", which at the discretion of the power company, may provide credit to the customer for the back-fed power. The most common source for the DC energy are photovoltaic modules or small wind turbines. Also commonly called "grid-interactive" inverter(s).

Grid tied

A solar electric system that's connected to the utility power grid and uses the utility grid as a backup source of power. If more energy is needed than is being generated by the solar electric system, the difference is supplied to the customer by the utility company. If more energy is being produced than needed, the excess electrical power flows backwards through the electrical meter to the utility company, where it is then used by others.





Heating Degree-Days

The amount of building heating needed, created by adding up all temperature differences of the form ( 65F - daily temperature in degrees F ) for each day in which the temperature falls below 65F. See also "cooling degree days".


"HERS", an abbreviation for "Home Energy Rating Systems", provides a standardized evaluation of a home's energy efficiency and expected energy costs. A home energy rating can qualify a home owner or home buyer for an energy efficient mortgage (EEM) or an energy improvement mortgage (EIM). A HERS Report is similar to getting a miles-per-gallon report on a car - but in this case, lower is better (lower energy usage means a more efficient home). To find a HERS Certified Home Energy Rater, click here.

Hybrid System

A heating system or energy generator that does not rely on one exclusive energy source. For example, many domestic photovoltaic systems use gasoline generators as backup systems during periods of extreme cloudiness. Utility-scale hybrid solar thermal power plants use sunlight to boil water, and an alternative energy source such as natural gas when there is insufficient sunlight.






Solar radiation in the visible region of the solar spectrum to which the human eye responds.

Incandescent Bulb

A light source that produces light by heating a wire filament to a very high temperature.

Incremental Capacity

Electrical generating capacity added on an annual basis.


A measure of the solar radiation energy reaching the earth in a given region over a specific period of time. Not to be confused with "insulation" - material that slows the movement of heat toward cold (such as in attics). Insolation is related to irradiance in a manner similar to how miles-per-hour are related to miles-per-day. Irradiance is the amount of solar energy falling on the earth's surface at any given moment. Insolation relates to the amount of energy over a longer period of time.


Materials that keep energy from moving from one place to another: on electrical wire, it is the plastic or rubber that covers the conductor; in a building, insulation makes the walls, floor, and roof more resistant to the outside (ambient) temperature.


An electrical device that changes direct current ("DC") into alternating current ("AC").


The rate at which solar radiant energy arrives on a specific area of surface during a given time interval. The typical unit is watts per square meter W/m2.






Joule is a derived unit of energy in the International System of Units equal to the energy expended (or work done) when an electric current of one ampere passes through a resistance of one ohm for one second. Named after the English physicist James Prescott Joule. Some mechanical definitions for Joule also apply, but are not included here.

Junction box

An electrical box designed to be a safe enclosure in which to make electrical connections. On photovoltaic modules, the junction box is on the back of the module, and is where the connections from the module and the wires are attached. Other electronics may also be present in the junction box.





Kilowatt ("kW")

One thousand watts of electricity. (See "Watt".)

Kilowatt-hour ("kW-h")

Frequently written "KWH" or "kwh". One thousand watt-hours. Calculated by multiplying the number of watts being used times the length of time in hours that amount of electricity is used. A refrigerator that uses 250 watts will consume one kilowatt-hour of energy in four hours (250 watts x 4 hours = 1,000 watt-hours .. or one kilowatt-hour). Utility bills are based on the number of kilowatt-hours consumed each month. One kilowatt-hour is equivalent to 3412 btu of energy. (See "btu".)





Landfill Gas

Gas that is generated by decomposition of organic material at landfill disposal sites. The average composition of landfill gas is approximately 50% methane and 50% carbon dioxide and water vapor by volume. The methane in landfill gas may be vented, flared, or combusted to generate electricity or useful thermal energy on-site, or injected into a pipeline for combustion off-site.


Abbreviation for "Leadership in Energy and Environmental Design".


Abbreviation for "LEED Accredited Professional".
Holders of a LEED AP Certification have passed an exam administered by the Green Building Certification Institute, a third-party organization that provides independent oversight of professional credentialing and project certification programs related to green building.
Through tests and examinations monitored by the GBCI, LEED AP have proven their understanding and knowledge of the LEED Rating System, green design, construction, and operations. Specialty certifications also exist for new construction, schools, retail establishments, commerical interiors, existing building operation and maintenance, and neighborhood development.

LEED Rating Systems

LEED Green Building Rating System is a third-party certification program and the nationally accepted benchmark for the design, construction and operation of high performance green buildings. LEED provides building owners and operators with the tools they need to have an immediate and measureable impact on their buildings' performance. LEED certification provides independent, third-party verification that a building project is environmentally responsible, profitable and a healthy place to live and work. The LEED Green Building Rating System encourages and accelerates global adoption of sustainable green building and development practices through the creation and implementation of universally understood and accepted tools and performance criteria.
The LEED rating system offers four certification levels for new construction -- Certified, Silver, Gold and Platinum -- that correspond to the number of "credits" (think of them as "points") accrued in five green design categories: sustainable sites, water efficiency, energy and atmosphere, materials and resources, and indoor environmental quality. LEED standards cover new commercial construction and major renovation projects, interiors projects and existing building operations.

Light Emitting Diode

An extremely efficient source of light, "LED" lamps convert from 65% to 95% of the electric energy to light energy (depending on the color of the light). LEDs also typically last 50,000 to 100,000 hours. Light emitting diodes are made from the same material as transistors and give off light when electricity is passed through them.






See "Modified Accelerated Cost Recovery System".


From "mega", meaning million, and "watt", a unit of energy (see "watt"). A megawatt is one million watts of electrical energy. To give you an idea how large one megawatt is, it's the energy consumed by 10 thousand 100 watt light bulbs illuminated at the same time.


One thousand kilowatt-hours, or 1 million watt-hours.


A complete small inverter (typically 200 to 300 watts) that's attached to the frame or rack-mount hardware of a photovoltaic module. Micro-inverters connect to the existing DC wiring of a photovoltaic module, and by requirement of the National Electric Code and Underwriters Laboratories Safety Standards, must contain ground-fault detection circuits that disable the micro-inverter in the event of a detected DC-side ground fault. See also "AC Module".

Modified Accelerated Cost Recovery System

Also known by the abbreviation "MACRS" (pronounced "makers"). Officially, the Federal Modified Accelerated Cost Recovery System. Under this system, a businesses may recover investments in certain property through rapid depreciation deductions. The MACRS establishes a set of classes for various types of property, ranging from three to 50 years, over which the property may be depreciated. A number of renewable energy technologies are classified as five-year property (26 USC § 168(e)(3)(B)(vi)) under the MACRS, which refers to 26 USC § 48(a)(3)(A), often known as the "Energy Investment Tax Credit" (or "ITC") to define eligible property.
Such property currently includes a variety of solar-electric and solar-thermal technologies, small wind turbines (100 kW or less), and a variety of other renewable and alternative power sources. For more information on the federal MACRS, see IRS Publication 946, IRS Form 4562: Depreciation and Amortization, and Instructions for Form 4562.

Monocrystalline Cells

Monocrystalline photovoltaic ("PV") cells are cut from cylindrical ingots that resemble a large round salami. They tend to convert more sunlight energy to electricity than other types of photovoltaic cells, but they are also the most expensive. Crystalline PV cells are made from silicon, the same material we use to make window glass, and typically have a very dark-to-black appearance over the entire surface of each cell.

Municipal Solid Waste

Residential solid waste and some nonhazardous commercial, institutional, and industrial wastes.






Abbreviation for "North American Board of Certified Energy Practitioners".
NABCEP is a volunteer board of renewable energy stakeholder representatives whose mission is to support, and work with the renewable energy and energy efficiency industries, professionals, and stakeholders to develop and implement quality credentialing and certification programs for practitioners (better known as solar energy installers).

Nameplate Capacity

The maximum rated output of a generator under specific conditions designated by the manufacturer. Nameplate capacity is usually indicated in units of kilovolt-amperes (kVA) and in kilowatts (kW) on a nameplate physically attached to the generator.

Net Metering

A contractual arrangement that permits an electrical utility customer to turn their electric meter backwards and sell any excess power generated (over and above their usage requirement) back to the electrical grid to offset some, to all of their consumption. Depending on individual state or utility rules, the net excess generation may be credited to their account (in many cases at the retail price), carried over to a future billing period, or ignored.

Net Zero

A "net-zero" building is generally considered to be connected to commercial utility power, and produces as much energy as it consumes. Most commonly, the energy sources are sunlight and/or wind. Buildings achieve "net-zero" status through a combination of energy-efficiency measures and on-site generation of electricity. Net-Zero buildings produce energy during the day, feeding any excess power to the utility grid, and use energy from the grid at night. An extensive review of "net-zero" buildings is offered by the U.S. National Renewable Energy Laboratory ("NREL"). NREL defines "net-zero" structures in a report titled "Zero Energy Buildings: A Critical Look at the Definition.". In their definition, NREL looks at where the energy is produced, and then how the energy is measured.

Nominal Operating Cell Temperature ("NOCT")

Also called "Normal Operating Cell Temperature". When exposed to sunlight, photovoltaic equipment operates at temperatures higher than the surrounding air. NOCT is the estimated temperature of the cells within a PV module when operating under 800 watts per square irradiance, 20 degrees C ambient temperature, and a wind speed of 1 meter per second (2.2 miles per hour). NOCT is used to estimate the nominal operating temperature of a module in its working environment.


Abbreviation for the National Renewable Energy Laboratory in Golden, Colorado. NREL is a United States National Laboratory specializing in developing technologies and procedures for using renewable energy sources.





Ocean Energy

Energy conversion technologies that harness the energy in tides, waves, and thermal gradients in the oceans.


An abbreviation for "over-current protection device". In plain language, this would be a circuit breaker, a fuse, or other device that opens an electrical circuit in the event too much current is flowing, such as too large of a load for the wiring .. or a short-circuit.

Off-the-grid ("Off-grid")

Not connected to the commercial power lines.

Open Circuit

A break in an electrical wire or other path where electricity would normally flow. An open circuit prevents any electrical current from flowing. Examples of an "open circuit" would be a broken wire, or even something like a wall or other switch that's "open" such that no current flows.






The official National Electric Code term for "breaker panel", "breaker box", or similar terms. An electrical distribution board that houses electrical circuit breakers. A panelboard is the main point at which electricity is distributed throughout a building. In commercial installations, it's often termed "electrical cabinet".

Passive Solar

Technology for using sunlight to illuminate and heat buildings directly, with no circulating fluid or energy conversion system, and usually no moving parts. Sunlight admitted into your home in the winter that helps to heat the house is a considered a "passive" energy source.


"Light-generated voltage". ("Photo" means "light". "Voltaic" (vol-TAY-ick) means "voltage". It's difficult to prounounce .. which is why we say "PV".

Photovoltaic ("PV") Cell

An electronic device consisting of layers of special materials capable of converting light directly into electricity.

Photovoltaic ("PV") Module

An assembly of interconnected photovoltaic cells enclosed in a protective assembly (usually glass and plastic).

Photovoltaic ("PV") Temperature Coefficient

Photovoltaic modules are affected by temperature. Contrary to what many people would think, PV efficiency goes UP in cold weather, and DOWN in hot weather. The rate at which the PV is affected by changes in temperature is known as its "temperature coefficient".

Polycrystalline Cells

Also called "poly-silicon" or "multi-crystalline" cells. Polycrystalline cells are made from square silicon ingots. Poly-cells are less expensive to produce than cells formed from single crystals, but are lower in efficiency (convert less of the sun's energy to electricity). Polycrystalline cells are easily distingushed by their mosaic blue appearance.


The amount of work or energy expended in a given amount of time. Typical units of power are "watts", joules, and calories".

Power Purchase Agreement

A contract between a power producer and a power consumer, which states that the customer will purchase a certain amount of power at a certain price from the producer.

Pumped-Storage Hydroelectric Plant

A plant that usually generates electric energy during peak load periods by using water previously pumped into an elevated storage reservoir during off-peak periods when excess generating capacity is available to do so.


See "Photovoltaic".


A device for measuring and reporting the amount of solar irradiance striking the earth's surface at any given moment. See also "irradiance".






No entries (yet) for the 'Q' category. Do you have a suggested/requested 'Q' word or phrase for our Glossary? Contact us!






See "Renewable Energy Credits".


"Resistance value". Used specifically for insulating materials to indicated its effectiveness against the movement of heat toward cold. The higher the number, the greater the slowing of the flow of energy. Three inches of fiberglass insulation has an R value of 7.5.


Mechanical structures that attache a photovoltaic or other solar energy array to the roof of a building, or to the ground.

Radiant Barrier

A layer of foil-type material typically attached to the underside of roof decking or attached to attic rafters. Radiant barrier consists of highly reflective material such as aluminum foil or other metal that reflects (or more specifically, re-emits) radiant heat rather than absorbing it. Radiant barriers don't, however, reduce heat conduction like thermal insulation. Radiant barriers have a products have a reflectivity of 95%-97%. Radiant barriers are more effective in hot climates than in cool climates, especially when cooling air ducts are located in the attic. Radiant barrier is not insulation, and will not prevent heat-loss in cold weather. Paint-on materials are not radiant barriers, in spite of frequent advertising claims to the contrary.

Renewable Electricity

Renewable electricity is electricity generated without use of fossil fuels.

Renewable Energy ("RE")

An energy source that renews itself without effort. Fossil fuels, once consumed, are gone forever, while solar energy is renewable in that the sun energy we harvest today has no effect on the sun energy we can harvest tomorrow. See also "biomass".

Renewable Energy Credits ("RECs")

Renewable energy credits, or RECs, are tradable commodities that represent the green attributes associated with energy generated from renewable energy resources. One "REC" is generated every time one megawatt-hour (MWh) of renewable electricity is produced. Solar renewable energy credits, or "SRECs" are RECs generated from solar power sources.
SRECs are valuable because some states have passed legislation requiring energy suppliers to provide a certain percentage of their electricity from renewable energy sources. This type of legislation is called a "Renewable Portfolio Standard" ("RPS"). Under certain types of Renewable Portfolio Standard legislation, energy suppliers must either generate a certain amount of solar electricity, or purchase SRECs to avoid paying a fine.

Renewable Energy Resources

Energy resources that are naturally replenishing but flow-limited. They are virtually inexhaustible in duration but limited in the amount of energy that is available per unit of time. Renewable energy resources include: biomass, hydro (water), geothermal, solar, wind, and ocean energy.


Shorthand term for "renewable energy" or easily renewable material sources.

Renewable Portfolio Standard ("RPS")

A renewable portfolio standard (RPS) requires that energy suppliers in a certain state produce a proportion of their energy from renewable energy. To meet these RPS requirements, energy suppliers can: 1) Develop their own renewable energy facilities such as solar energy power plants or wind farms to produce RECS, or; 2) Purchase RECs from others that own renewable energy facilities.





Short Circuit

A "short circuit" is an unwanted path for electrical current between two points in a circuit. In the case of electrical power, a short-circuit usually allows excessive current to flow (more than the wires are designed to handle), in which case a circuit breaker or fuse will open the circuit, stopping the current flow, and thus preventing damage and/or a fire. (See also "OCPD").


Semiconductor material made from silica (beach sand), which is then purified for photovoltaic applications.


Abbreviation for "State of Charge" - a reference to the amount of energy stored in a battery. A 100% state of charge ("SOC") means the battery is fully charged and cannot accept more. A 50% SOC means the battery has delivered half of its available energy. 0% SOC means the battery is totally discharged.

Solar Cell

A device made of silicon and other materials. Solar cells generate electricity when exposed to sunlight. Multiple solar cells (typically from 36 to 96 cells) are used in the construction of one photovoltaic module.

Solar Collector
(Solar Thermal Collector)

A solar collector is that part of the system which absorbs the sun's energy and converts it into heat, such as a solar collector for a solar hot water system. Solar collectors can convert typically up to 85% of the sun's energy to heat. Not to be confused with a photovoltaic module, sometimes called a "solar panel". The heat collected by the solar collector may be used immediately or stored for later use. Solar collectors are used for space heating; domestic hot water heating; and heating swimming pools, hot tubs, or spas.

Solar Energy

The radiant energy of the sun, which can be converted into other forms of energy such as heat or electricity.

Solar Modules

Also called "solar panels". These are the large collections of solar cells that can produce electricity in a worthwhile quantity.

Solar Noon

The time of day when the sun reaches its highest point in the sky. This time divides the daylight hours for that day exactly in half. Solar noon may be quite a bit different from 'clock' noon depending on the location relative to the time zone in which the measurement is taken.

Solar Renewable Energy Credits

SREC program provides a means for Solar Renewable Energy Certificates (SRECs) to be created for every megawatt-hour of solar electricity created.

Solar Thermal Electric Generation (STEG)

Conversion of solar energy to electricity using various technologies to heat a working fluid to power a turbine that drives a generator. Examples of these systems include central receiver systems, parabolic dish, and solar trough.


See "Solar Renewable Energy Credits".

Standard Test Conditions ("STC")

"Standard Test Conditions". An attempt by the renewable energy industry, and photovoltaic manufacturing in particular, to provide consistent test conditions for photovoltic performance. STC is based on an irradiance of 1,000 watts per square meter of sunlight, a solar spectrum of air mass 1.5, and a module temperature of 25C.


"Sustainability" relates to the quality of life in a community, and is frequently used as meaning not taking more from a resource than is replenshed naturally. In a broader sense, sustainability pertains to the economic, social and environmental systems that make up a community as a whole, and whether all three are working in concert to provide a healthy, productive, meaningful life for all community residents, present and future.





Thermal Solar

The process of deriving or concentrating heat from sunlight. Examples of "derived heat" are: home heating, solar cooking, clothes drying, solar heated water, and so forth. Concentrated solar thermal heat is often used to create steam, from which electric power is generated.

Thin Film

Thin-film photovoltaic modules are manufactured using spray-on or print-on techniques, rather than forming cells from ingots or blocks of molten silicon. Thin-film photovoltaic modules are significantly lower in efficiency than crystalline PV, but are also lower in cost per watt of energy generated, and often less affected by heat. Thin-film technologies involve many types of metals including silicon, gallium, cadmium, tellurium, copper, and others.

Time-of-Use Metering
("TOU Metering")

A utility billing system in which the price of electricity depends upon the hour of day at which it is used. Rates are typically higher during the afternoon when electric demand is at its peak. Rates are lower during at night when electric demand is much less.


Abbreviation for "Typical Meteorological Year" -- a "typical" year of hourly solar and meteorological values which is designed to produce the expected climate of a location throughout a year. TMY data for years 1961 through 1990 ("TMY 2") is available from the National Solar Radiation TMY2 Database. TMY data for 1991 through 2005 ("TMY 3") is available from the National Solar Radiation TMY3 Database.


Mechanical device used in solar electric and solar thermal systems. Follows the movement of the sun (daily and sometimes seasonally) and keeps the energy collection device pointed directly at the sun. Allows for the harvest of the maximum available solar energy.

Tracking Collector/Tracker

Any mechanical structure that that changes its orientation throughout the day in order to follow the path of the sun in the sky. Two-axis trackers continually face the sun throughout the day, changing direction with the time of day and the season. Single-axis trackers rotate on one axis for time-of-day only, and must be manually adjusted for the season. See also "tracker".





Uninterruptible Power Supply ("UPS")

A device (usually containing batteries) that stores power for use when conventional power is unavailable, such as during a power failure.





Volatile Organic Compound ("VOC")

Volatile organic compounds are organic compounds that contain carbon. VOCs have a boiling point below that of water, and easily vaporize (or "volatilize"). VOCs are solvents that get released into the air, such as when paint dries. Other products that emit VOCs include adhesives, cleaning supplies, and even some home furnishings. A few examples of VOCs are formaldehyde, toluene, acetone, and benzene. Products may be described as "low-VOC" when they emit significantly less than other products, but they still emit potentially harmful substances, which can accumulate in enclosed or poorly ventilated areas.


Unit of electrical pressure. Think of "volts" as if it were water pressure.






Unit of electrical power. "Watts" are calculated by multiplying the electrical pressure in a circuit ("volts") by the amount of electricity moving in the circuit ("amps"). For example, 120 volts times 2 amps equals 240 watts.

Watt-hour ("W-h")

The electrical energy unit of measure equal to one watt of power supplied to, or taken from, an electric circuit steadily for one hour. For instance, a 60 watt incandescent light will consume 600 watt-hours of energy when used for ten hours (60 watts x 10 hours = 600 watt-hours.). Our electric bills are based on the number of watt-hours of energy consumed each month.

Wind Energy

Kinetic energy present in wind motion that can be converted to mechanical energy for driving pumps, mills, and electric power generators.

Wind Turbine

Also called "Wind Generator" or incorrectly "windmill". Devices consisting of blades that turn a shaft that turns a generator to harvest wind energy and produce electrical power.






"Xeriscape" is a combination of the Greek word "xeros", meaning "dry", and "-scape," as in landscape. Xeriscape landscaping essentially refers to a creating a landscape design that's low-water use, and tailored to withstand drought conditions.

Pronounced as if it begins with the letter 'z' (as in "xylophone"), use of the word "xeriscaping" originated with the Denver Colorado Water Department in 1981. Also known as "low-water-use" landscaping.






No entries (yet) for the 'Y' category. Do you have a suggested/requested 'Y' word or phrase for our Glossary? Contact us!






At the "zenith" in the sky, the sun will be directly overhead in relation to the observer.

Zenith Angle

The angle between the direction of interest (of the sun, for example) and the zenith (directly overhead). See also "zenith".





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