Sacrificial Anode
A metal rod placed in a water heater tank to protect the tank from corrosion. Anodes of aluminum, magnesium, or zinc are the more frequently metals. The anode creates a galvanic cell in which magnesium or zinc will be corroded more quickly than the metal of the tank giving the tank a negative charge and preventing corrosion.

Safety Disconnect

An electronic (automatic or manual) switch that disconnects one circuit from another circuit. These are used to isolate power generation or storage equipment from conditions such as voltage spikes or surges, thus avoiding potential damage to equipment.

Salt Gradient Solar Ponds

Consist of three main layers. The top layer is near ambient and has low salt content. The bottom layer is hot, typically 160 F to 212 F (71 C to 100 C), and is very salty. The important gradient zone separates these zones. The gradient zone acts as a transparent insulator, permitting the sunlight to be trapped in the hot bottom layer (from which useful heat is withdrawn). This is because the salt gradient, which increases the brine density with depth, counteracts the buoyancy effect of the warmer water below (which would otherwise rise to the surface and lose its heat to the air). An organic Rankine cycle engine is used to convert the thermal energy to electricity.


The cutting of a grid pattern of grooves in a semiconductor material, generally for the purpose of making interconnections.

Sealed Combustion Heating System

A heating system that uses only outside air for combustion and vents combustion gases directly to the outdoors. These systems are less likely to backdraft and to negatively affect indoor air quality.

Seasonal Energy Efficiency Ratio (SEER)

A measure of seasonal or annual efficiency of a central air conditioner or air conditioning heat pump. It takes into account the variations in temperature that can occur within a season and is the average number of Btu of cooling delivered for every watt-hour of electricity used by the heat pump over a cooling season.

Seasonal Performance Factor (SPF)

Ratio of useful energy output of a device to the energy input, averaged over an entire heating season.

Seasoned Wood

Wood, used for fuel, that has been air dried so that it contains 15 to 20 percent moisture content (wet basis).

Seebeck Effect

The generation of an electric current, when two conductors of different metals are joined at their ends to form a circuit, with the two junctions kept at different temperatures.

Second Law Efficiency

The ratio of the minimum amount of work or energy required to perform a task to the amount actually used.

Second Law of Thermodynamics

This law states that no device can completely and continuously transform all of the energy supplied to it into useful energy.

Selectable Load

Any device, such as lights, televisions, and power tools, which is plugged into your central power source and used only intermittently.

Selective Absorber

A solar absorber surface that has high absorbence at wavelengths corresponding to that of the solar spectrum and low emittance in the infrared range.

Selective Surface Coating

A material with high absorbence and low emittance properties applied to or on solar absorber surfaces.


Any material that has a limited capacity for conducting an electric current. Certain semiconductors, including silicon, gallium arsenide, copper indium diselenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process.

Sensible Cooling Effect

The difference between the total cooling effect and the dehumidifying effect.

Sensible Cooling Load

The interior heat gain due to heat conduction, convection, and radiation from the exterior into the interior, and from occupants and appliances.

Sensible Heat

The heat absorbed or released when a substance undergoes a change in temperature.

Sensible Heat Storage

A heat storage system that uses a heat storage medium, and where the additional or removal of heat results in a change in temperature.


A configuration of an electrical circuit in which the positive lead is connected to the negative lead of another energy producing, conducting, or consuming device. The voltages of each device are additive, whereas the current is not.

Series Connection

A way of joining photovoltaic cells by connecting positive leads to negative leads; such a configuration increases the voltage.

Series Resistance

Parasitic resistance to current flow in a cell due to mechanisms such as resistance from the bulk of the semiconductor material, metallic contacts, and interconnections.

Setback Thermostat

A thermostat that can be set to automatically lower temperatures in an unoccupied house and raise them again before the occupant returns.

Shading Coefficient

A measure of window glazing performance that is the ratio of the total solar heat gain through a specific window to the total solar heat gain through a single sheet of double-strength glass under the same set of conditions; expressed as a number between 0 and 1.


A construction element used to cover the exterior of wall framing and roof trusses.

Short Circuit

An electric current taking a shorter or different path than intended.

Short Circuit Current

The current flowing freely through an external circuit that has no load or resistance; the maximum current possible.

Shunt Load

An electrical load used to safely use excess generated power when not needed for its primary uses. A shunt load in a residential photovoltaic system might be domestic water heating, such that when power is not needed for typical building loads, such as operating lights or running HVAC system fans and pumps, it still provides value and is used in a constructive, safe manner.


An interior or exterior movable panel that operates on hinges or slides into place, used to protect windows or provide privacy.


A construction element applied to the outermost surface of an exterior wall.

Sigma Heat

The sum of sensible heat and latent heat in a substance above a base temperature, typically 32 degrees Fahrenheit.


A chemical element, of atomic number 14, that is semi-metallic, and an excellent semiconductor material used in solar photovoltaic devices; commonly found in sand.

Simple CS (Caulk and Seal)

A technique for insulating and sealing exterior walls that reduces vapor diffusion through air leakage points by installing pre-cut blocks of rigid foam insulation over floor joists, sheet subfloor, and top plates before drywall is installed.

Sine Wave

The type of alternative current generated by alternating current generators, rotary inverters, and solid-state inverters.

Single-Crystal Material

In reference to solar photovoltaic devices, a material that is composed of a single crystal or a few large crystals.

Single Glaze or Pane

One layer of glass in a window frame. It has very little insulating value (R-1) and provides only a thin barrier to the outside and can account for considerable heat loss and gain.

Single-Package System

A year ’round heating and air conditioning system that has all the components completely encased in one unit outside the home. Proper matching of components can mean more energy-efficient operation compared to components purchased separately.


A generator with a single armature coil, which may have many turns and the alternating current output consists of a succession of cycles.


The process of designing a solar system to meet a specified load given the solar resource and the nominal or rated energy output of the solar energy collection or conversion device.


A window located on the roof of a structure to provide interior building spaces with natural daylight, warmth, and ventilation.


A concrete pad that sits on gravel or crushed rock, well-compacted soil either level with the ground or above the ground.

Slab on Grade

A slab floor that sits directly on top of the surrounding ground.

TM Ground Loop
In this type of closed-loop, horizontal geothermal heat pump installation, the fluid-filled plastic heat exchanger pipes are coiled like a SlinkyTM to allow more pipe in a shorter trench. This type of installation cuts down on installation costs and makes horizontal installation possible in areas it would not be with conventional horizontal applications. Also see closed-loop geothermal heat pump systems.

Smart Window

A term used to describe a technologically advanced window system that contains glazing that can change or switch its optical qualities when a low voltage electrical signal is applied to it, or in response to changes in heat or light.

Sodium Lights

A type of high intensity discharge light that has the most lumens per watt of any light source.


A panel which covers the underside of an roof overhang, cantilever, or mansard.

Solar Access or Rights

The legal issues related to protecting or ensuring access to sunlight to operate a solar energy system, or use solar energy for heating and cooling.

Solar Altitude Angle

The angle between a line from a point on the earth’s surface to the center of the solar disc, and a line extending horizontally from the point.

Solar Air Heater

A type of solar thermal system where air is heated in a collector and either transferred directly to the interior space or to a storage medium, such as a rock bin.

Solar Array

A group of solar collectors or solar modules connected together.

Solar Azimuth

The angle between the sun’s apparent position in the sky and true south, as measured on a horizontal plane.

Solar Cell

A solar photovoltaic device with a specified area.

Solar Collector

A device used to collect, absorb, and transfer solar energy to a working fluid. Flat plate collectors are the most common type of collectors used for solar water or pool heating systems. In the case of a photovoltaics system, the solar collector could be crystalline silicon panels or thin-film roof shingles, for example.

Solar Constant

The average amount of solar radiation that reaches the earth’s upper atmosphere on a surface perpendicular to the sun’s rays; equal to 1353 Watts per square meter or 492 Btu per square foot.

Solar Cooling

The use of solar thermal energy or solar electricity to power a cooling appliance. There are five basic types of solar cooling technologies: absorption cooling, which can use solar thermal energy to vaporize the refrigerant; desiccant cooling, which can use solar thermal energy to regenerate (dry) the desiccant; vapor compression cooling, which can use solar thermal energy to operate a Rankine-cycle heat engine; and evaporative coolers (“swamp” coolers), and heat-pumps and air conditioners that can by powered by solar photovoltaic systems.

Solar Declination

The apparent angle of the sun north or south of the earth’s equatorial plane. The earth’s rotation on its axis causes a daily change in the declination.

Solar Distillation

The process of distilling (purifying) water using solar energy. Water can be placed in an air tight solar collector with a sloped glazing material, and as it heats and evaporates, distilled water condenses on the collector glazing, and runs down where it can be collected in a tray.

Solar Energy

Electromagnetic energy transmitted from the sun (solar radiation). The amount that reaches the earth is equal to one billionth of total solar energy generated, or the equivalent of about 420 trillion kilowatt-hours.

Solar Energy Collector

See solar collector.

Solar Energy Industries Association (SEIA)

A national trade association of solar energy equipment manufacturers, retailers, suppliers, installers, and consultants.

Solar Energy Research Institute (SERI)

A federally funded institute, created by the Solar Energy Research, Development and Demonstration Act of 1974, that conducted research and development of solar energy technologies. Became the National Renewable Energy Laboratory (NREL) in 1991.

Solar Film

A window glazing coating, usually tinted bronze or gray, used to reduce building cooling loads, glare, and fabric fading.

Solar Fraction

The percentage of a building’s seasonal energy requirements that can be met by a solar energy device(s) or system(s).

Solar Furnace

A device that achieves very high temperatures by the use of reflectors to focus and concentrate sunlight onto a small receiver.

Solar Gain

The amount of energy that a building absorbs due to solar energy striking its exterior and conducting to the interior or passing through windows and being absorbed by materials in the building.

Solar Irradiation

The amount of solar radiation, both direct and diffuse, received at any location.


A glazed structure, such as greenhouse or “sunspace.”

Solar Mass

A term used for materials used to absorb and store solar energy.

Solar Module (Panel)

A solar photovoltaic device that produces a specified power output under defined test conditions, usually composed of groups of solar cells connected in series, in parallel, or in series-parallel combinations.

Solar Noon

The time of the day, at a specific location, when the sun reaches its highest, apparent point in the sky; equal to true or due, geographic south.

Solar One

A solar thermal electric central receiver power plant (“power tower”) located in Barstow, California, and completed in 1981. The Solar One had a design capacity of 10,000 peak kilowatts, and was composed of a receiver located on the top of a tower surrounded by a field of reflectors. The concentrated sunlight created steam to drive a steam turbine and electric generator located on the ground.

Solar Pond

A body of water that contains brackish (highly saline) water that forms layers of differing salinity (stratifies) that absorb and trap solar energy. Solar ponds can be used to provide heat for industrial or agricultural processes, building heating and cooling, and to generate electricity.

Solar Power Satellite

A solar power station investigated by NASA that entailed a satellite in geosynchronous orbit that would consist of a very large array of solar photovoltaic modules that would convert solar generated electricity to microwaves and beam them to a fixed point on the earth.

Solar Panel

See Photovoltaic Module.

Solar Radiation

A general term for the visible and near visible (ultraviolet and near-infrared) electromagnetic radiation that is emitted by the sun. It has a spectral, or wavelength, distribution that corresponds to different energy levels; short wavelength radiation has a higher energy than long-wavelength radiation.

Solar Simulator

An apparatus that replicates the solar spectrum, and used for testing solar energy conversion devices.

Solar Space Heater

A solar energy system designed to provide heat to individual rooms in a building.

Solar Spectrum

The total distribution of electromagnetic radiation emanating from the sun. The different regions of the solar spectrum are described by their wavelength range. The visible region extends from about 390 to 780 nanometers (a nanometer is one billionth of one meter). About 99 percent of solar radiation is contained in a wavelength region from 300 nm (ultraviolet) to 3,000 nm (near-infrared). The combined radiation in the wavelength region from 280 nm to 4,000 nm is called the broadband, or total, solar radiation.

Solar Thermal Electric Systems

Solar energy conversion technologies that convert solar energy to electricity, by heating a working fluid to power a turbine that drives a generator. Examples of these systems include central receiver systems, parabolic dish, and solar trough.

Solar Thermal Parabolic Dishes

A solar thermal technology that uses a modular mirror system that approximates a parabola and incorporates two-axis tracking to focus the sunlight onto receivers located at the focal point of each dish. The mirror system typically is made from a number of mirror facets, either glass or polymer mirror, or can consist of a single stretched membrane using a polymer mirror. The concentrated sunlight may be used directly by a Stirling, Rankine, or Brayton cycle heat engine at the focal point of the receiver or to heat a working fluid that is piped to a central engine. The primary applications include remote electrification, water pumping, and grid-connected generation.

Solar Thermal Systems

Solar energy systems that collect or absorb solar energy for useful purposes. Can be used to generate high temperature heat (for electricity production and/or process heat), medium temperature heat (for process and space/water heating and electricity generation), and low temperature heat (for water and space heating and cooling).

Solar Time

The period marked by successive crossing of the earth’s meridian by the sun; the hour angle of the sun at a point of observance (apparent time) is corrected to true (solar) time by taking into account the variation in the earth’s orbit and rate of rotation. Solar time and local standard time are usually different for any specific location.

Solar Trough Systems (see also Parabolic Trough, above)

A type of solar thermal system where sunlight is concentrated by a curved reflector onto a pipe containing a working fluid that can be used for process heat or to produce electricity. The world’s largest solar thermal electric power plants use solar trough technology. They are located in California, and have a combined electricity generating capacity of 240,000 kilowatts.

Solar Transmittance

The amount of solar energy that passes through a glazing material, expressed as a percentage.

Solar Two

Solar Two is a retrofit of the Solar One project (see above). It is demonstrating the technical feasibility and power potential of a solar power tower using advanced molten-salt technology to store energy. Solar Two retains several of the main components of Solar One, including the receiver tower, turbine, generator, and the
1,818 heliostats.


An electromechanical device composed of a coil of wire wound around a cylinder containing a bar or plunger, that when a current is applied to the coil, the electromotive force causes the plunger to move; a series of coils or wires used to produce a magnetic field.

Solenoid Valve

An automatic valve that is opened or closed by an electromagnet.

Solid Fuels

Any fuel that is in solid form, such as wood, peat, lignite, coal, and manufactured fuels such as pulverized coal, coke, charcoal, briquettes, pellets, etc.


In reference to a wind energy conversion device, the ratio of rotor blade surface area to the frontal, swept area that the rotor passes through.


The two times of the year when the sun is apparently farthest north and south of the earth’s equator; usually occurring on or around June 21 (summer solstice in northern hemisphere, winter solstice for southern hemisphere) and December 21 (winter solstice in northern hemisphere, summer solstice for the southern hemisphere).

Space Heater

A movable or fixed heater used to heat individual rooms.

Spacer (Window)

Strips of material used to separate multiple panes of glass within the windows.

Specific Heat

The amount of heat required to raise a unit mass of a substance through one degree, expressed as a ratio of the amount of heat required to raise an equal mass of water through the same range.

Specific Heat Capacity

The quantity of heat required to change the temperature of one unit weight of a material by one degree.

Specific Humidity

The weight of water vapor, per unit weight of dry air.

Specific Volume

The volume of a unit weight of a substance at a specific temperature and pressure.

Spectral Energy Distribution

A curve illustrating the variation or spectral irradiance with wavelength.

Spectral Irradiance

The monochromatic irradiance of a surface per unit bandwidth at a particular wavelength, usually expressed in Watts per square meter-nanometer bandwidth.

Spectral Reflectance

The ratio of energy reflected from a surface in a given waveband to the energy incident in that waveband.


see Solar Spectrum above.

Spectrally Selective Coatings

A type of window glazing films used to block the infrared (heat) portion of the solar spectrum but admit a higher portion of visible light.


A passage for surplus water to flow over or around a dam.

Spinning Reserve

Electric power provider capacity on line and running at low power in excess of actual load.

Split Spectrum Photovoltaic Cell

A photovoltaic device where incident sunlight is split into different spectral regions, with an optical apparatus, that are directed to individual photovoltaic cells that are optimized for converting that spectrum to electricity.

Split System Air Conditioner

An air conditioning system that comes in two to five pieces: one piece contains the compressor, condenser, and a fan; the others have an evaporator and a fan. The condenser, installed outside the house, connects to several evaporators, one in each room to be cooled, mounted inside the house. Each evaporator is individually controlled, allowing different rooms or zones to be cooled to varying degrees.

Spray Pyrolysis

A deposition process whereby heat is used to break molecules into elemental sources that are then spray deposited on a substrate.

Spreader Stocker

A type of furnace in which fuel is spread, automatically or mechanically, across the furnace grate.


A process used to apply photovoltaic semi-conductor material to a substrate by a physical vapor deposition process where high-energy ions are used to bombard elemental sources of semiconductor material, which eject vapors of atoms that are then deposited in thin layers on a substrate.

Square Wave Inverter

A type of inverter that produces square wave output.; consists of a DC source, four switches, and the load. The switches are power semiconductors that can carry a large current and withstand a high voltage rating. The switches are turned on and off at a correct sequence, at a certain frequency. The square wave inverter is the simplest and the least expensive to purchase, but it produces the lowest quality of power.

Squirrel Cage Motor

This is another name for an induction motor. The motors consist of a rotor inside a stator. The rotor has laminated, thin flat steel discs, stacked with channels along the length. If the casting composed of bars and attached end rings were viewed without the laminations the casting would appear similar to a squirrel cage.

Staebler-Wronski effect

The tendency of the sunlight to electricity conversion efficiency of amorphous silicon photovoltaic devices to degrade (drop) upon initial exposure to light.


A smokestack or flue for exhausting the products of combustion from a combustion appliance.

Stack (Heat) Loss

Sensible and latent heat contained in combustion gases and vapor emitted to the atmosphere.

Stagnation Temperature

A condition that can occur in a solar collector if the working fluid does not circulate when sun is shining on the collector.


In reference to a wind turbine, a condition when the rotor stops turning.

Stand-Alone Generator

A power source/generator that operates independently of or is not connected to an electric transmission and distribution network; used to meet a load(s) physically close to the generator.

Stand-Alone Inverter

An inverter that operates independent of or is not connected to an electric transmission and distribution network.

Stand-Alone System

An system that operates independent of or is not connected to an electric transmission and distribution network.

Standard Air

Air with a weight of 0.075 pounds per cubic foot with an equivalent density of dry air at a temperature of 86 degrees Fahrenheit and standard barometric pressure of 29.92 inches of mercury.

Standard Conditions

In refrigeration, an evaporating temperature of 5 degrees Fahrenheit (F), a condensing temperature of 86 degrees F., liquid temperature before expansion of 77 degrees F., and suction temperature of 12 degrees F.

Standard Cubic Foot

A column of gas at standard conditions of temperature and pressure (32 degrees Fahrenheit and one atmosphere).

Standard Industrial Classification (SIC) Code

Standardized codes used to classify businesses by type of activity they engage in.

Stand-by Heat Loses

A term used to describe heat energy lost from a water heater tank.

Stand-By Power

For the consumer, this is the electricity that is used by your TVs, stereos, and other electronic devices that use remote controls. When you press “off” to turn off your device, minimal power (dormant mode) is still being used to maintain the internal electronics in a ready, quick-response mode. This way, your device can be turned on with your remote control and be immediately ready to operate.

Static Pressure

The force per unit area acting on the surface of a solid boundary parallel to the flow.

Starting Surge

Power, often above an appliance’s rated wattage, required to bring any appliance with a motor up to operating speed.

Starting Torque

The torque at the bottom of a speed (rpm) versus torque curve. The torque developed by the motor is a percentage of the full-load or rated torque. At this torque the speed, the rotational speed of the motor as a percentage of synchronous speed is zero. This torque is what is available to initially get the load moving and begin its acceleration.


Water in vapor form; used as the working fluid in steam turbines and heating systems.

Steam Boiler

A type of furnace in which fuel is burned and the heat is used to produce steam.

Steam Turbine

A device that converts high-pressure steam, produced in a boiler, into mechanical energy that can then be used to produce electricity by forcing blades in a cylinder to rotate and turn a generator shaft.

Stirling Engine

A heat engine of the reciprocating (piston) where the working gas and a heat source are independent. The working gas is compressed in one region of the engine and transferred to another region where it is expanded. The expanded gas is then returned to the first region for recompression. The working gas thus moves back and forth in a closed cycle.


Chemical reactions, typically associated with combustion processes; the balancing of chemical reactions by providing the exact proportions of reactant compounds to ensure a complete reaction; all the reactants are used up to produce a single set of products.

Stoichiometric Ratio

The ratio of chemical substances necessary for a reaction to occur completely.

Storage Capacity

The amount of energy an energy storage device or system can store.

Storage Hydropower

A hydropower facility that stores water in a reservoir during high-inflow periods to augment water during low-inflow periods. Storage projects allow the flow releases and power production to be more flexible and dependable. Many hydropower project operations use a combination of approaches.

Storage Tank

The tank of a water heater.

Storage Water Heater

A water heater that releases hot water from the top of the tank when a hot water tap is opened. To replace that hot water, cold water enters the bottom of the tank to ensure a full tank.

Storm Door

An exterior door that protects the primary door.

Storm Windows

Glass, plastic panels, or plastic sheets that reduce air infiltration and some heat loss when attached to either the interior or exterior of existing windows.

Stranded Investment (Costs and Benefits)

An investment in a power plant or demand side management measures or programs, that become uneconomical due to increased competition in the electric power market. For example, an electric power plant may produce power that is more costly than what the market rate for electricity is, and the power plant owner may have to close the plant, even though the capital and financing costs of building the plant have not been recovered through prior sales of electricity from the plant. This is considered a Stranded Cost. Stranded Benefits are those power provider investments in measures or programs considered to benefit consumers by reducing energy consumption and/or providing environmental benefits that have to be curtailed due to increased competition and lower profit margins.


A popular term used for a length of wood or steel used in or for wall framing.


An electrical installation containing power conversion (and sometimes generation) equipment, such as transformers, compensators, and circuit breakers.


The physical material upon which a photovoltaic cell is applied.

Sun Path Diagram

A circular projection of the sky vault onto a flat diagram used to determine solar positions and shading effects of landscape features on a solar energy system.


A room that faces south (in the northern hemisphere), or a small structure attached to the south side of a house.

Sun Tempered Building

A building that is elongated in the east-west direction, with the majority of the windows on the south side. The area of the windows is generally limited to about 7% of the total floor area. A sun-tempered design has no added thermal mass beyond what is already in the framing, wall board, and so on. Insulation levels are generally high.

Superconducting Magnetic Energy Storage (SMES)

SMES technology uses the superconducting characteristics of low-temperature materials to produce intense magnetic fields to store energy. SMES has been proposed as a storage option to support large-scale use of photovoltaics and wind as a means to smooth out fluctuations in power generation.


The abrupt and large increase in electrical conductivity exhibited by some metals as the temperature approaches absolute zero.

Super Insulated Houses

A type of house that has massive amounts of insulation, airtight construction, and controlled ventilation without sacrificing comfort, health, or aesthetics.

Super Window

A popular term for highly insulating window with a heat loss so low it performs better than an insulated wall in winter, since the sunlight that it admits is greater than its heat loss over a 24 hour period.

Supplementary Heat

A heat source, such as a space heater, used to provide more heat than that provided by a primary heating source.

Supply Duct

The duct(s) of a forced air heating/cooling system through which heated or cooled air is supplied to rooms by the action of the fan of the central heating or cooling unit.

Supply Side

Technologies that pertain to the generation of electricity.

Surface Water Loop

In this type of closed-loop geothermal heat pump installation, the fluid-filled plastic heat exchanger pipes are coiled into circles and submerged at least eight feet below the surface of a body of surface water, such as a pond or lake. The coils should only be placed in a water source that meets minimum volume, depth, and quality criteria. Also see closed-loop geothermal heat pump systems.

Swamp Cooler

A popular term used for an evaporative cooling device.

Swept Area

In reference to a wind energy conversion device, the area through which the rotor blades spin, as seen when directly facing the center of the rotor blades.

Synchronous Generator

An electrical generator that runs at a constant speed and draws its excitation from a power source external or independent of the load or transmission network it is supplying.

Synchronous Inverter

An electrical inverter that inverts direct current electricity to alternating current electricity, and that uses another alternating current source, such as an electric power transmission and distribution network (grid), for voltage and frequency reference to provide power in phase and at the same frequency as the external power source.

Synchronous Motor

A type of motor designed to operate precisely at the synchronous speed with no slip in the full-load speeds (rpm).

System Mix

The proportion of electricity distributed by a power provider that is generated from available sources such as coal, natural gas, petroleum, nuclear, hydropower, wind, or geothermal.