In wastewater treatment or livestock facilities, a shallow pond used to store wastewater where sunlight and biological activity decompose the waste.


A light source composed of a metal base, a glass tube filled with an inert gas or a vapor, and base pins to attach to a fixture.


Features and vegetation on the outside of or surrounding a building for aesthetics and energy conservation.


A unit or measure of solar radiation; 1 calorie per square centimeter or 3.69 Btu per square foot.


The regular periodic arrangement of atoms or molecules in a crystal of semiconductor material.

Latent Cooling Load

The load created by moisture in the air, including from outside air infiltration and that from indoor sources such as occupants, plants, cooking, showering, etc.

Latent Heat

The change in heat content that occurs with a change in phase and without change in temperature.

Latent Heat of Vaporization

The quantity of heat produced to change a unit weight of a liquid to vapor with no change in temperature.

Law(s) of Thermodynamics

The first law states that energy can not be created or destroyed; the second law states that when a free exchange of heat occurs between two materials, the heat always moves from the warmer to the cooler material.

Lead Acid Battery

An electrochemical battery that uses lead and lead oxide for electrodes and sulfuric acid for the electrolyte.

Leading Edge

In reference to a wind energy conversion system, the area of a turbine blade surface that first comes into contact with the wind.

Leaking Electricity

Related to stand-by power, leaking electricity is the power needed for electrical equipment to remain ready for use while in a dormant mode or operation. Electricity is still used by many electrical devices, such as TVs, stereos, and computers, even when you think they are turned “off.”


A measure of air purity that is equal to one complete air change (in an interior space).

Levelized Life Cycle Cost

A total life cycle cost divided into equal amounts.

Life Cycle Cost

The sum of all the costs both recurring and nonrecurring, related to a product, structure, system, or service during its life span or specified time period.


The force that pulls a wind turbine blade, as opposed to drag.

Light Quality

A description of how well people in a lighted space can see to do visual tasks and how visually comfortable they feel in that space.

Light-Induced Defects

Defects, such as dangling bonds, induced in an amorphous silicon semiconductor upon initial exposure to light.

Light Trapping

The trapping of light inside a semiconductor material by refracting and reflecting the light at critical angles; trapped light will travel further in the material, greatly increasing the probability of absorption and hence of producing charge carriers.

Line Loss (or Drop)

Electrical energy lost due to inherent inefficiencies in an electrical transmission and distribution system under specific conditions.

Liquid-Based Solar Heating System

A solar heating system that uses a liquid as the heat transfer fluid.

Liquid-To-Air Heat Exchanger

A heat exchanger that transfers the heat contained in a liquid heat transfer fluid to air.

Liquid-To-Liquid Heat Exchanger

A heat exchanger that transfers heat contained in a liquid heat transfer fluid to another liquid.

Lithium-Sulfur Battery

A battery that uses lithium in the negative electrode and a metal sulfide in the positive electrode, and the electrolyte is molten salt; can store large amounts of energy per unit weight.

Live Steam

Steam available directly from a boiler under full pressure.


The power required to run a defined circuit or system, such as a refrigerator, building, or an entire electricity distribution system.

Load Analysis

Assessing and quantifying the discrete components that comprise a load. This analysis often includes time of day or season as a variable.

Load Duration Curve

A curve that displays load values on the horizontal axis in descending order of magnitude against the percent of time (on the vertical axis) that the load values are exceeded.

Load Factor

The ratio of average energy demand (load) to maximum demand (peak load) during a specific period.

Load Forecast

An estimate of power demand at some future period.

Load Leveling

The deferment of certain loads to limit electrical power demand, or the production of energy during off-peak periods for storage and use during peak demand periods.

Load Management

To influence the demand on a power source.

Load Profile or Shape

A curve on a chart showing power (kW) supplied (on the horizontal axis) plotted against time of occurrence (on the vertical axis) to illustrate the variance in a load in a specified time period.

Load Shedding

Turning off or disconnecting loads to limit peak demand.

Load Shifting

A load management objective that moves loads from on-peak periods to off-peak periods.

Local Solar Time

A system of astronomical time in which the sun crosses the true north-south meridian at 12 noon, and which differs from local time according to longitude, time zone, and equation of time.

Log Law

In reference to a wind energy conversion system, the wind speed profile in which wind speeds increase with the logarithmic of the height of the wind turbine above the ground.

Long Ton

A unit that equals 20 long hundredweight or 2,240 pounds. Used mainly in England.

Long-Wave Radiation

Infrared or radiant heat.

Loose Fill Insulation

Insulation made from rockwool fibers, fiberglass, cellulose fiber, vermiculite or perlite minerals, and composed of loose fibers or granules can be applied by pouring directly from the bag or with a blower.

Losses (Energy)

A general term applied to the energy that is converted to a form that can not be effectively used (lost) during the operation of an energy producing, conducting, or consuming system.

Loss of Load Probability (LOLP)

A measure of the probability that a system demand will exceed capacity during a given period; often expressed as the estimated number of days over a long period, frequently 10 years or the life of the system.

Low Btu Gas

A fuel gas with a heating value between 90 and 200 Btu per cubic foot.

Low-E Coatings & (Window) Films

A coating applied to the surface of the glazing of a window to reduce heat transfer through the window.

Low-Emissivity Windows & (Window) Films

Energy-efficient windows that have a coating or film applied to the surface of the glass to reduce heat transfer through the window.

Lower (Net) Heating Value

The lower or net heat of combustion for a fuel that assumes that all products of combustion are in a gaseous state. (See Net Heating Value below.)

Low-Flow Solar Water Heating Systems

The flow rate in these systems is 1/8 to 1/5 the rate of most solar water heating systems. The low-flow systems take advantage of stratification in the storage tank and theoretically allows for the use of smaller diameter piping to and from the collector and a smaller pump.

Low Flush Toilet

A toilet that uses less water than a standard one during flushing, for the purpose of conserving water resources.

Low-Pressure Sodium Lamp

A type of lamp that produces light from sodium gas contained in a bulb operating at a partial pressure of 0.13 to 1.3 Pascal. The yellow light and large size make them applicable to lighting streets and parking lots.


An empirical measure of the quantity of light. It is based upon the spectral sensitivity of the photosensors in the human eye under high (daytime) light levels. Photometrically it is the luminous flux emitted with a solid angle (1 steradian) by a point source having a uniform luminous intensity of 1 candela.

Lumens/Watt (lpw)

A measure of the efficacy (efficiency) of lamps. It indicates the amount of light (lumens) emitted by the lamp for each unit of electrical power (Watts) used.


A complete lighting unit consisting of a lamp(s), housing, and connection to the power circuit.


The physical measure of the subjective sensation of brightness; measured in lumens.


The unit of illuminance equivalent to 1lumen per square meter.