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Energy Efficiency Dictionary

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Adjustable speed drive

A device which promotes more efficient energy use by matching energy output with demand. Fans and pumps are the most familiar applications associated with this technology. For example, a fan driven by a fixed-speed motor (one which does not take demand into account) will sometimes produce more airflow than is needed, wasting energy. A fan with an adjustable speed drive regulates the speed of the motor to produce only the amount of airflow that is necessary.

Air sealing

The important counterpart to insulation (see “insulation”), air sealing is designed to plug holes or cracks in a home. Even a well-insulated home can suffer air leakage if there are small gaps around doors and windows, holes in the basement, attic or crawlspace, or leaks in the duct-work. Sealing these leaks can make a big difference in a home's comfort and energy bills; though a small degree of ventilation is necessary to ensure proper air quality.

Availability Based Tariff (ABT)

A frequency-based electricity pricing mechanism that provides a system of incentives and disincentives to encourage peak-hour energy production. Improving the stability of power supply in this way avoids sharp fluctuations in frequency, which damage electrical equipment, and prevents power outages and strain on the grid.



An abbreviation for British thermal unit. The term is used to describe the energy content of fuels, and also to describe the power of heating and cooling systems, such as furnaces, stoves, barbecue grills and air conditioners. As a common unit of measurement, it offers a practical means to talk about energy that comes from multiple sources. A Btu can be approximated as the heat produced by burning a single wooden match.


A piece of equipment used to control the starting and operating voltages of electrical gas discharge lights, such as fluorescent and neon lights and high-intensity discharge (HID) lamps. There are three primary types of ballasts: magnetic (the least sophisticated type); and electronic and digital (both more efficient options). Electronic ballasts offer many benefits including longer average bulb life, reduced light flicker and reduced ballast hum. Also, since electronic ballasts produce much less heat than magnetic ballasts, they can save energy by reducing the load on the air-conditioning equipment.


Biogases include the methane and carbon dioxide produced in fermentation and anaerobic digestion of biomass, municipal and biological wastes, as well as the nitrogen, hydrogen, carbon monoxide and methane produced by the gasification of woody biomass. It can be used to produce electricity, such as in sewage works or CHP plants, and can be compressed for more efficient use in vehicles.

Biomass Energy

Derived from plant materials, waste, landfill gas, and alcohol fuels, biomass is used to produce electricity or thermal energy usually through direct incineration. Biomass also is used to make biofuels, such as biogas or biodiesel, after a biochemical conversion such as composting or transesterification.

Building energy codes

A subset of a broader group of construction standards known as building codes. Building energy codes set construction standards for insulation levels (see insulation); window and door specifications (see windows); heating, ventilation, and air-conditioning equipment efficiency (see HVAC); and lighting fixtures and controls (see lighting). By reducing the amount of energy that buildings consume, building energy codes reduce energy use and save consumers money on their energy bills. The Building Codes Assistance Project has more information about building energy codes.

Building Automation System (BAS)

A control system based on a network of electronic devices that monitor and schedule lighting, temperature control, air circulation, water heating and chilling, and security to optimize energy use and maintain a healthy indoor environment.

Building envelope

A term for the outer shell of a structure, the building envelope is what separates the indoor air from the outdoor air. In the winter, a good building envelope will keep warm air in the house and cold air out; in summer, it will do the opposite. An energy audit (see “energy audit”) will likely include a thorough look at the building envelope. Sealing cracks and holes (see “air sealing”) and properly installing insulation (see “insulation”) are ways to keep a building envelop in good condition.


Combined Heat and Power (CHP)

An energy-generating system that provides on-site integrated thermal and electricity production with waste-heat recovery. Steam turbines, gas turbines, or biomass can fuel the system, producing either heat or electricity as the byproduct. CHP systems are more fuel-efficient because of heat-recovery and they avoid distribution losses thanks to on-site generation.

Compact Fluorescent Lamp (CFL)

Despite higher embodied energy, CFLs use about a quarter of the energy of traditional incandescent bulbs and have a lifespan eight to 15 times longer. Integrated CFLs — with built-in electronic ballasts — can be exchanged directly with incandescent bulbs. Some non-integrated CFLs still have magnetic ballasts, which cause the tubes to flicker when first switched on.

Compact fluorescent light bulb (CFL)

A more efficient version of a traditional light bulb. A CFL can produce the same amount of light as incandescent bulbs, but it uses less energy to do so. A CFL costs about 75 percent less to run, paying for itself in about six months and lasting up to ten years.

Constant Air Volume (CAV)

See Variable Air Volume


Demand billing

A system of charging consumers for energy use based on their highest rate of consumption during a given period. It is usually applied to industrial and commercial consumers – not residential energy users. Under demand billing, throughout a billing period, a demand meter periodically records the level of power that a customer is using. Customers are charged a certain amount for their total energy consumed, and an additional amount based on their highest level of demand, even though they may operate at this level for only one short period during a month’s billing cycle.

Distributed generation

The process of generating energy from small, decentralized sources, such as solar photovoltaic cells and wind turbines. This approach reduces the amount of energy lost in transmitting electricity because the electricity is generated very near where it is used, sometimes even in the same building. In addition, distributed generation minimizes transmission and distribution costs which represent a significant part of electricity’s total cost.


Energy audit (Home Performance Assessment)

A process that determines a home or building's efficiency, and recommends energy-saving measures that can improve efficiency. An audit can be performed by a home improvement contractor, a utility representative or other professional.

Energy efficiency retro-fit

Any home improvement project that increases the energy efficiency of a home. Common energy efficiency retrofit projects include adding insulation to the building envelope (see “building envelope“); sealing (see “air sealing”) ducts and leaks; and replacing an HVAC system with high-efficiency equipment. Like other home improvement projects, energy efficiency retrofits increase the value of a home while also making it more comfortable, with the added benefit of reducing the homeowner's energy bills.

Energy Management Software (EMS)

A type of software that collects energy-use data, as well as sets energy conservation goals, monitors real-time energy consumption, and provides an interface through which users manage energy use and identify savings opportunities. EMS also incorporates models that detect abnormalities in energy use.

Energy performance contract

An Energy performance contract is an agreement between a building owner and a private energy services company (ESCO) that uses future energy savings to pay for the cost of a building’s energy efficiency retrofits. A building owner contracts with an ESCO, which then finances, designs, purchases, installs and maintains energy-saving improvements to the building. As the building accrues savings on its utility bills, the ESCO recoups its investment.

Energy tax credit

A government incentive to encourage consumers to make purchases that will improve their energy efficiency. Current federal energy tax incentives cover fuel-efficient hybrid-electric or diesel vehicles and specified energy efficiency upgrades to existing homes.


Fuel-Efficient Wood Stove

Rocket Stoves are characterized by a combined oxygen intake and fuel input opening, as well as a combustion chamber, chimney, and heat exchanger. More complete combustion and directed heat channelization achieve greater combustion efficiency. A Lion Stove feeds air underneath the fuel, from the opposite direction. The fuel blocks the flames from the stove walls, maximizing heat transfer upwards, instead of to the bricks.


Geothermal Heat Pump (GHP)

A system that utilizes the relatively constant underground temperatures year-round to transfer heat to a building in the winter and vice versa in the summer. This heating and cooling method is clean, requires little energy, and is inexpensive to operate.

Geothermal Power Plant

Mostly flash steam plants, which take hot water from inside the Earth, use it as steam to power generators, and inject it back into the geothermal reservoir. Dry steam plants use steam directly from the earth to turn the turbines. Binary power plants pump hot water from the geothermal reservoir, transferring the heat to convert another liquid to steam, which powers the generators.



Incandescent light bulb

The “traditional” light bulb that has been in use for over a century. In recent years, polices and a general market shift have begun to favor more energy-efficient options such as compact fluorescent lamps (see CFLs), high-intensity discharge lamps, light-emitting diodes (see LEDs). These alternatives produce the same amount of light as incandescent bulbs, but with less energy. The United States has committed to phasing out incandescent bulbs from the marketplace within the next several years.


Any of a variety of materials used to “pad” a home, and keep warm air in and cold air out. The U.S. Environmental Protection Agency (U.S. EPA) ENERGY STAR program estimates that, when used together, insulation and air sealing (see “air sealing”) can reduce heating and cooling bills by up to 20 percent.




Light Emitting Diode (LEDs)

A more energy-efficient, longer-lasting alternative to traditional lighting options. Unlike incandescent bulbs, LEDs do not have a filament. Instead, an LED’s light is produced by a semiconductor diode. Since approximately 90 percent of an incandescent light’s power use involves heating the filament, this is one principal way in which LEDs are more efficient.





Photovoltaics (PVs)

PV modules or “panels” are interconnected PV cells through which electrons excited by the photovoltaic effect produce direct current energy directly from sunlight. PVs are not to be confused with solar thermal, or concentrating solar power (CSP).

Pressure Management (in water systems)

The most common method to reduce water system losses and demand in water systems. Generally, reducing water system losses and demand is achieved by decreasing water pressure in supply pipes, anticipating surges, and optimizing altitude changes along delivery lines. Pressure reduction establishes distribution zones; pump control; fixed outlet, time- and flow-modulated control valves; and remote node control.




Solar Panel

See Photovoltaic’s.

Solar Thermal or Concentrating Solar Power (CSP)

Reflects the sun’s energy with mirror panels or concave dishes that concentrate heat, as well as power a conventional steam turbine to produce electricity. Not to be confused with photovoltaics.


Tax credit

See “energy tax credit.”

Thermal Storage Tank

Off-Peak Cooling tanks use a chilled eutectic mixture to freeze ice at night during off-peak hours; the thermal storage tank then uses this ice to chill water for air conditioning during the day. Chillers can be 30 to 40 percent smaller due to reduced peak energy use. Heat storage also is an application of thermal storage tanks for use in space and water heating, or electricity generation. Sometimes solar collectors are used to produce the initial heat.



Variable Air Volume (VAV)

Air handler that uses a VSD to control the speed of a fan blowing air of a constant temperature into a single thermal zone, such as a room or office. VAVs are more efficient than Constant Air Volume (CAV) handlers. CAVs blow a fixed volume of air, varying the temperature with internal heat exchangers, and typically serve thermal zones with a greater area.

Variable Frequency Drive (VFD)

A device that varies the electricity input of an electric motor according to the demands of the application. VSDs save electricity and energy costs, and preserve the life of the equipment. VSDs apply to AC motors and also vary the voltage of DC motors.

Variable Speed Drive (VSD)

See Variable Frequency Drive.


Wind Turbine

Consists of a rotor, which converts wind energy into low-speed rotational energy; a generator with gears that produce high-rotational speed to generate electric energy; and the support system. Wind energy is the fastest-growing electricity source and has the potential to gain significant market shares in the next decade. Maintenance costs are the greatest hindrance to expansion in the sector.





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