Molten or partially molten rock at temperatures ranging from 1,260 F to 2,880 F (700 C to 1600 C). Some magma bodies are believed to exist at drillable depths within the Earth’s crust, although practical technologies for harnessing magma energy have not been developed. If ever utilized, magma represents a potentially enormous resource.

Magnetic Ballast

A type of florescent light ballast that uses a magnetic core to regulate the voltage of a florescent lamp.

Make-Up Air

Air brought into a building from outside to replace exhaust air.

Manual J

The standard method for calculating residential cooling loads developed by the Air-Conditioning and Refrigeration Institute (ARI) and the Air Conditioning Contractors of America (ACCA) based largely on the American Society of Heating, Refrigeration, and Air-Conditioning Engineer’s (ASHRAE) “Handbook of Fundamentals.”

Marginal Cost

The cost of producing one additional unit of a product.


Material such as brick, rock, or stone.

Masonry Stove

A type of heating appliance similar to a fireplace, but much more efficient and clean burning. They are made of masonry and have long channels through which combustion gases give up their heat to the heavy mass of the stove, which releases the heat slowly into a room. Often called Russian or Finnish fireplaces.

Mass Burn Facility

A type of municipal solid waste (MSW) incineration facility in which MSW is burned with only minor presorting to remove oversize, hazardous, or explosive materials. Mass burn facilities can be large, with capacities of 3000 tons (2.7 million kg) of MSW per day or more. They can be scaled down to handle the waste from smaller communities, and modular plants with capacities as low as 25 tons (22.7 thousand kg) per day have been built. Mass burn technologies represent over 75% of all the MSW-to-energy facilities constructed in the United States to date. The major components of a mass burn facility include refuse receiving and handling, combustion and steam generation, flue gas cleaning, power generation (optional), condenser cooling water, residue ash hauling and landfilling.


An abbreviation for one thousand cubic feet of natural gas with a heat content of 1,000,000 Btus, or 10 therms.

Mean Power Output (of a Wind Turbine)

The average power output of a wind energy conversion system at a given mean wind speed based on a Raleigh frequency distribution.

Mean Wind Speed

The arithmetic wind speed over a specified time period and height above the ground (the majority of U.S. National Weather Service anemometers are at 20 feet (6.1 meters).

Mechanical Systems

Those elements of building used to control the interior climate.

Median Wind Speed

The wind speed with 50 percent probability of occurring.

Medium Btu Gas

Fuel gas with a heating value of between 200 and 300 Btu per cubic foot.

Medium Pressure

For valves and fittings, implies that they are suitable for working pressures between 125 to 175 pounds per square inch.


One thousand kilowatts, or 1 million watts; standard measure of electric power plant generating capacity.


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

Mercury Vapor Lamp

A high-intensity discharge lamp that uses mercury as the primary light-producing element. Includes clear, phosphor coated, and self-ballasted lamps.


An approximate unit of heat produced by a resting person, equal to about 18.5 Btu per square foot per hour.

Metal Halide Lamp

A high-intensity discharge lamp type that uses mercury and several halide additives as light-producing elements. These lights have the best Color Rendition Index (CRI) of the High-Intensity Discharge lamps. They can be used for commercial interior lighting or for stadium lights.


A colorless, odorless, tasteless gas composed of one molecule of Carbon and four of hydrogen, which is highly flammable. It is the main constituent of “natural gas” that is formed naturally by methanogenic, anaerobic bacteria or can be manufactured, and which is used as a fuel and for manufacturing chemicals.

Methanol (CH
3OH; Methyl alcohol or wood alcohol)
A clear, colorless, very mobile liquid that is flammable and poisonous; used as a fuel and fuel additive, and to produce chemicals.

Metric Ton (Tonne)

A unit of mass equal to 1,000 kilograms or 2,204.6 pounds.


The local climate of specific place or habitat, as influenced by landscape features.


A small groove scribed into the surface of a solar photovoltaic cell which is filled with metal for contacts.


One millionth of a meter (10-6 m).


A common monetary measure equal to one-thousandth of a dollar or a tenth of a cent.

Minority Carrier

A current carrier, either an electron or a hole, that is in the minority in a specific layer of a semiconductor material; the diffusion of minority carriers under the action of the cell junction voltage is the current in a photovoltaic device.

Minority Carrier Lifetime

The average time a minority carrier exists before recombination.

Mixing Valve

A valve operated by a thermostat that can be installed in solar water heating systems to mix cold water with water from the collector loop to maintain a safe water temperature.

Modified Degree-Day Method

A method used to estimate building heating loads by assuming that heat loss and gain is proportional to the equivalent heat-loss coefficient for the building envelope.


The smallest self-contained, environmentally protected structure housing interconnected photovoltaic cells and providing a single dc electrical output; also called a panel.

Moisture Content

The water content of a substance (a solid fuel) as measured under specified conditions being the: Dry Basis, which equals the weight of the wet sample minus the weight of a (bone) dry sample divided by the weight of the dry sample times 100 (to get percent); Wet Basis, which is equal to the weight of the wet sample minus the weight of the dry sample divided by the weight of the wet sample times 100.

Moisture Control

The process of controlling indoor moisture levels and condensation.


Fabricated as a single structure.


The planting, cultivation, and harvesting of a single species of crop in a specified area.


A machine supplied with external energy that is converted into force and/or motion.

Motor Speed

The number of revolutions that the motor turns in a given time period (i.e. revolutions per minute, rpm).

Movable Insulation

A device that reduces heat loss at night and during cloudy periods and heat gain during the day in warm weather. A movable insulator could be an insulative shade, shutter panel, or curtain.


Methyl Tertiary Butyl Ether (MTBE) is an ether compound used as a gasoline blending component to raise the oxygen content of gasoline. MTBE is made by combining isobutylene (from various refining and chemical processes) and methanol (usually made from natural gas).

Multijunction Device

A high-efficiency photovoltaic device containing two or more cell junctions, each of which is optimized for a particular part of the solar spectrum.

Multi-Zone System

A building heating, ventilation, and/or air conditioning system that distributes conditioned air to individual zones or rooms.

Municipal Solid Waste (MSW)

Waste material from households and businesses in a community that is not regulated as hazardous.

Municipal Waste

As defined in the Energy Security Act (P.L. 96-294; 1980) as “any organic matter, including sewage, sewage sludge, and industrial or commercial waste, and mixtures of such matter and inorganic refuse from any publicly or privately operated municipal waste collection or similar disposal system, or from similar waste flows (other than such flows which constitute agricultural wastes or residues, or wood wastes or residues from wood harvesting activities or production of forest products).”

Municipal Waste to Energy Project (or Plant)

A facility that produces fuel or energy from municipal solid waste.