To heat a gaseous mixture to the temperature at which combustion takes place.

Ignition Point

The minimum temperature at which combustion of a solid or fluid can occur.


A measure of the amount of light incident on a surface; measured in foot-candles or Lux.


A body of water confined by a dam, dike, floodgate or other artificial barrier.

Impulse Turbine

A turbine that is driven by high velocity jets of water or steam from a nozzle directed to vanes or buckets attached to a wheel. (A pelton wheel is an impulse hydro turbine).


These lights use an electrically heated filament to produce light in a vacuum or inert gas-filled bulb.

Incident Solar Radiation

The amount of solar radiation striking a surface per unit of time and area.

Independent Power Producer

A company or individual that is not directly regulated as a power provider. These entities produce power for their own use and/or sell it to regulated power providers.

Indirect Solar Gain System

A passive solar heating system in which the sun warms a heat storage element, and the heat is distributed to the interior space by convection, conduction, and radiation.

Indirect Solar Water Heater

These systems circulate fluids other than water (such as diluted antifreeze) through the collector. The collected heat is transferred to the household water supply using a heat exchanger. Also known as “closed-loop” systems.


The production of an electric current in a conductor by the variation of a magnetic field in its vicinity.

Induction Generator

A device that converts the mechanical energy of rotation into electricity based on electromagnetic induction. An electric voltage (electromotive force) is induced in a conducting loop (or coil) when there is a change in the number of magnetic field lines (or magnetic flux) passing through the loop. When the loop is closed by connecting the ends through an external load, the induced voltage will cause an electric current to flow through the loop and load. Thus rotational energy is converted into electrical energy.

Induction Motor

A motor in which a three phase (or any multiphase) alternating current (i.e. the working current) is supplied to iron-cored coils (or windings) within the stator. As a result, a rotating magnetic field is set up, which induces a magnetizing current in the rotor coils (or windings). Interaction of the magnetic field produced in this manner with the rotating field causes rotational motion to occur.

Industrial Process Heat

The thermal energy used in an industrial process.

Inert Gas

A gas that does not react with other substances; e.g. argon or krypton; sealed between two sheets of glazing to decrease the U-value (increase the R-Value) of windows.

Infrared Radiation

Electromagnetic radiation whose wavelengths lie in the range from 0.75 micrometer to 1000 micrometers; invisible long wavelength radiation (heat) capable of producing a thermal or photovoltaic effect, though less effective than visible light.


The solar power density incident on a surface of stated area and orientation, usually expressed as Watts per square meter or Btu per square foot per hour.

Installed Capacity

The total capacity of electrical generation devices in a power station or system.

Instantaneous Efficiency (of a Solar Collector)

The amount of energy absorbed (or converted) by a solar collector (or photovoltaic cell or module) over a 15 minute period.


Materials that prevent or slow down the movement of heat.

Insulation Blanket

A pre-cut layer of insulation applied around a water heater storage tank to reduce stand-by heat loss from the tank.


A device or material with a high resistance to electricity flow.

Integral Collector Storage System

This simple passive solar hot water system consists of one or more storage tanks placed in an insulated box that has a glazed side facing the sun. An integral collector storage system is mounted on the ground or on the roof (make sure your roof structure is strong enough to support it). Some systems use “selective” surfaces on the tank(s). These surfaces absorb sun well but inhibit radiative loss. Also known as bread box systems or batch heaters.

Integrated Heating Systems

A type of heating appliance that performs more than one function, for example space and water heating.

Integrated Resource Plan (IRP)

A plan developed by an electric power provider, sometimes as required by a public regulatory commission or agency, that defines the short and long term capacity additions (supply side) and demand side management programs that it will undertake to meet projected energy demands.


A connection or link between power systems that enables them to draw on each other’s reserve capacity in time of need.

Intermittent Generators

Power plants, whose output depends on a factor(s) that cannot be controlled by the power generator because they utilize intermittent resources such as solar energy or the wind.

Internal Combustion Electric Power Plant

The generation of electric power by a heat engine which converts part of the heat generated by combustion of the fuel into mechanical motion to operate an electric generator.

Internal Gain

The heat produced by sources of heat in a building (occupants, appliances, lighting, etc).

Internal Mass

Materials with high thermal energy storage capacity contained in or part of a building’s walls, floors, or freestanding elements.

Internal Rate of Return

A widely used rate of return for performing economic analysis. This method solves for the interest rate that equates the equivalent worth of an alternative’s cash receipts or savings to the equivalent worth of cash expenditures, including investments. The resultant interest rate is termed the internal rate of return (IRR).

Interruptible Load

Energy loads that can be shut off or disconnected at the supplier’s discretion or as determined by a contractual agreement between the supplier and the customer.

Intrinsic Layer

A layer of semiconductor material (as used in a solar photovoltaic device) whose properties are essentially those of the pure, undoped, material.


A device that converts direct current electricity (from for example a solar photovoltaic module or array) to alternating current for use directly to operate appliances or to supply power to a electricity grid.

Investment Tax Credit

A tax credit granted for specific types of investments.

Investor Owned Utility (IOU)

A power provider owned by stockholders or other investors; sometimes referred to as a private power provider, in contrast to a public power provider that is owned by a government agency or cooperative.


An electrically charged atom or group of atoms that has lost or gained electrons; a loss makes the resulting particle positively charged; a gain makes the particle negatively charged.


A device that removes airborne particles from breathable air. Negative ions are produced and give up their negative charge to the particles. These new negative particles are then attracted to the positive particles surrounding them. This accumulation process continues until the particles become heavy enough to fall to the ground.


The direct, diffuse, and reflected solar radiation that strikes a surface.

Isolated Solar Gain System

A type of passive solar heating system where heat is collected in one area for use in another.

I-Type Semiconductor

A semiconductor material that is left intrinsic, or undoped so that the concentration of charge carriers is characteristic of the material itself rather than of added impurities.

I-V Curve

A graphical plot or representation the current and voltage output of a solar photovoltaic cell or module as a load on the device is increased from short circuit (no load) condition to the open circuit condition; used to characterize cell/module performance.