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Absorptance: The ratio of the radiation absorbed by a surface to the total energy falling on that surface described as a percentage.

Active Solar Energy: Solar radiation used by special equipment to provide space heating, hot water or electricity.

Active Solar Energy System: A system designed to convert solar radiation into usable energy for space, water heating, or other uses. It requires a mechanical device, usually a pump or fan, to collect the sun's energy.

Addition: An alteration to an existing building that increases conditioned space.

Adjusted Coefficient of Performance (ACOP): A standard rating term used to rate the efficiency of heat pumps. ACOP was replaced by Heating Seasonal Performance Factor (HSPF) in 1988.

Adverse Hydro:Water conditions limiting the production of hydroelectric power. In years having below–normal levels of rain and snow, and in seasons having less–than–usual runoff from mountain snowpack, there is then less water available for hydro energy production.

Afforestation: Planting of new forests on lands that have not been recently forested.

After Market: A broad term that applies to any change after the original purchase, such as adding equipment not a part of the original purchase. As applied to alternative fuel vehicles, it refers to conversion devices or kits for conventional fuel vehicles.

Aggregator: An entity responsible for planning, scheduling, accounting, billing, and settlement for energy deliveries from the aggregator's portfolio of sellers and/or buyers. Aggregators seek to bring together customers or generators so they can buy or sell power in bulk, making a profit on the transaction.

Air Change: The replacement of a quantity of air in a space within a given period of time, typically expressed as air changes per hour. If a building has one air change per hour, this is equivalent to all of the air in the building being replaced in a one–hour period.

Air Conditioner: An assembly of equipment for air treatment consisting of a means for ventilation, air circulation, air cleaning, and heat transfer (either heating or cooling). The unit usually consists of an evaporator or cooling coil, and an electrically–driven compressor and condenser combination.

Air Film: A layer of still air adjacent to a surface which provides some thermal resistance.

Air Film Coefficient: A measure of the heat transfer through an air film. (ASHRAE Table 1, ASHRAE Handbook, 1985 Fundamentals)

Air–to–Air Heat Exchanger: A device with separate air chambers that transfers heat between the conditioned air being exhausted and the outside air being supplied to a building.

Air Pollution: Unwanted particles, mist or gases put into the atmosphere as a result of motor vehicle exhaust, the operation of industrial facilities or other human activity.

Alcohol Fuels: A class of liquid chemicals that have certain combinations of hydrogen, carbon and oxygen, and that are capable of being used as fuel.

Alteration: Any change or modification to a building's construction. See Addition.

Alternating Current (AC): Flow of electricity that constantly changes direction between positive and negative sides. Almost all power produced by electric utilities in the United States moves in current that shifts direction at a rate of 60 times per second.

Alternative Energy Sources: See Renewable Energy.

Alternative (transportation) Fuels as defined by the National Energy Policy Act (EPAct) the fuels are: methanol, denatured ethanol and other alcohols, separately or in mixtures of 85 percent by volume or more (or other percentage not less than 70 percent as determined by U.S. Department of Energy rule) with gasoline or other fuels; CNG; LNG; LPG; hydrogen; “coal–derived liquid fuels;” fuels “other than alcohols” derived from “biological materials;” electricity, or any other fuel determined to be “substantially not petroleum” and yielding “substantial energy security benefits and substantial environmental benefits.”

Alternative Fuel Vehicle (AFV): Motor vehicles that run on fuels other than petroleum–based fuels. As defined by the National Energy Policy Act EPAct, this excludes reformulated gasoline as an alternative fuel.

Ambient: The surrounding atmosphere; encompassing on all sides; the environment surrounding a body but undisturbed or unaffected by it.

Ambient Air Temperature: Surrounding temperature, such as the outdoor air temperature around a building.

American National Standards Institute (ANSI): is the national organization that coordinates development and maintenance of consensus standards and sets rules for fairness in their development. ANSI also represents the USA in developing international standards.

Ampere (AMP): The unit of measure that tells how much electricity flows through a conductor. It is like using cubic feet per second to measure the flow of water. For example, a 1,200 watt, 120–volt hair dryer pulls 10 amperes of electric current (watts divided by volts).

Ancillary Services: Services that the Independent System Operator may develop, in cooperation with market participants, to ensure reliability and to support the transmission of energy from generation sites to customer loads. Such services may include: regulation, spinning reserve, non–spinning reserve, replacement reserve, voltage support, and black start.

Angle of Incidence: The angle that the sun's rays make with a line perpendicular to a surface. The angle of incidence determines the percentage of direct sunshine intercepted by a surface.

Animal Waste Conversion: Process of obtaining energy from animal wastes. This is a type of biomass energy.

Annual Fuel Utilization Efficiency (AFUE): A measure of heating efficiency, in consistent units, determined by applying the federal test method for furnaces. This value is intended to represent the ratio of heat transferred to the conditioned space by the fuel energy supplied over one year.

Annual Maximum Demand: The greatest of all demands of the electrical load which occurred during a prescribed interval in a calendar year.

Anthracite Coal: Hard coal, found deep in the earth. It burns very hot, with little flame. It usually has a heating value of 12,000–15,000 British thermal units per pound.

Appliance Efficiency Standards: Since 1988, the U.S. Department of Energy has issued numerous efficiency standards requiring the manufacture of appliances to meet or exceed minimum levels of efficiency. In 1987, Congress passed the National Appliance Energy Conservation Act, which permitted the establishment of test procedures and efficiency standards for 13 consumer products. From 1988 to 1995, DOE established and revised efficiency standards almost on an annual basis. No standards were revised between 1996 and July 2000. After that date the federal energy agency established a new process. Since that time a number of efficiency standards have been revised.

Appliance Saturation: A percentage telling what proportion of all households in a given geographical area have a certain appliance.

Area Load: The total amount of electricity being used at a given point in time by all consumers in a utility's service territory.

Ash: Non–organic, non–flammable substance left over after combustible material has been completely burned.

ASHRAE: Acronym for American Society of Heating, Refrigerating and Air–Conditioning Engineers.

Asphalt: A dark–brown–to–black cement–like material containing bitumens as the predominant constituents. It is obtained by petroleum processing.

Associated Energy Losses: See Electrical System Energy Losses.

Associated Gas: Natural gas that can be developed for commercial use, and which is found in contact with oil in naturally occurring underground formations.

ATgas: Synthetic gas produced by dissolving coal in a bath of molten iron. The process was developed by Applied Technology, Inc. Synthetic gas may be used as a substitute for natural gas in industrial and home uses.

Atom: The smallest unit of an element consisting of a dense positively charged nucleus (of protons and neutrons) orbited by negatively charged electrons.

Atomic Energy Commission: The independent civilian agency of the federal government with statutory responsibility to supervise and promote use of nuclear energy. Functions were taken over in 1974 by the Energy Research and Development Administration (now part of the U.S. Department of Energy) and the Nuclear Regulatory Commission.

Atomic Nucleus: The positively charged core of an atom.

Auxiliary Energy Subsystem: Equipment using conventional fuel to supplement the energy output of a solar system. This might be, for example, an oil–fueled generator that adds to the electrical output of a substitutes for the solar system during long overcast periods when there is not enough sunlight.

Auxiliary Equipment: Extra machinery needed to support the operation of a power plant or other large facility.

Average Demand: The energy demand in a given geographical area over a period of time. For example, the number of kilowatthours used in a 24–hour period, divided by 24, tells the average demand for that period.

Average Hydro: Rain, snow and runoff conditions that provide water for hydroelectric generation equal to the most commonly occurring levels. Average hydro usually is a mean indicating the levels experienced most often in a 104–year period.

Aviation Fuel: A category of fuel that includes aviation gasoline and jet fuel.

Aviation Gasoline: All special grades of gasoline for use in aviation reciprocating engines.

Avoided Costs (Regulatory): The amount of money that an electric utility would need to spend for the next increment of electric generation to produce or purchase elsewhere the power that it instead buys from a cogenerator or small–power producer. Federal law establishes broad guidelines for determining how much a qualifying facility gets paid for power sold to the utility.

Azimuth: The angular distance between true south and the point on the horizon directly below the sun. Typically used as an input for opaque surfaces and windows in computer programs for calculating the energy performance of buildings.