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Glossary of Efficiency Terms
This glossary contains many of the efficiency terms used throughout this website. For definitions of any terms not found in this glossary, contact the Energy Services Office.
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AFUE
(Annual Fuel Utilization Efficiency) back to "Find the definition for . . ." |
. . . an efficiency rating that measures the efficiency with which natural gas and other fossil-fuel-burning furnaces and boilers use their primary fuel source over an entire heating season. It does not take into account the efficiency with which any component of the system, such as a furnace fan motor, uses electricity. AFUE is expressed as a percentage that indicates the average number of Btu worth of heating comfort provided by each Btu of natural gas (or other fossil fuel) consumed by the system. For instance, a gas furnace with an AFUE of 80% will provide 0.8 Btu of heat for every 1.0 Btu of natural gas it burns. Learn more about AFUE. |
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Air infiltration back to "Find the definition for . . ." |
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. . the introduction, usually unintentional, of unconditioned outdoor
air into a mechanically heated and/or cooled building. Air infiltration
can occur through any opening in the home's structure, including: seams
where walls meet other walls, window or door frames, or chimneys; holes
where wires or pipes penetrate walls, floors or ceilings/roofs; and the
space between loose-fitting meeting-rails of a double-hung window or
between the bottom of a door and the door's threshold. Along with internal heat gain and solar heat gain, air infiltration can play a significant role in the load that is placed on your building's heating and cooling equipment, as well as the comfort of the building's occupants. |
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Btu
(British Thermal Unit) back to "Find the definition for . . ." |
. . . a measurement of the energy contained in heat. It takes one Btu of heat to warm one pound of water by 1° Fahrenheit. Btu can be used either to define an air conditioner's cooling capacity (i.e., the number of Btu of heat that can be removed by the system) or a furnace's heating capacity (i.e., the number of Btu of heat that can be supplied by the system. |
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Caulk back to "Find the definition for . . ." |
. . . a substance used to seal air infiltration points between two immovable objects, such as where exterior or interior wall surfaces meet window or door frames and at corners formed by siding. Most caulks come in tubes and are applied with the use of a caulk "gun." |
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CFL
(Compact Fluorescent Lamp) back to "Find the definition for . . ." |
. . . a light "bulb" using fluorescent technology but designed to be used in many of the same fixtures traditionally used by standard incandescent "A" bulbs. CFLs incorporate a small-diameter looped or swirled tube that is attached to a screw-in base. CFLs provide light levels comparable to 20- to 150-watt incandescent bulbs for 70% to 75% less energy. They can also last up to 13 times longer than equivalent incandescent bulbs. |
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Conduction back to "Find the definition for . . ." |
. . . the transfer of heat through solid objects, such as glass, dry wall, brick and other building materials. The greater the difference between the outdoor and indoor temperatures, the faster conduction will occur and the more heat a building can gain or lose. |
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Convection back to "Find the definition for . . ." |
. . . the transfer of heat to or from a solid surface via a gaseous or liquid current. Where home heat loss and gain are concerned, heat convection is caused by air (gas) currents that carry heat from warm objects (such as your body, furniture, and interior walls) to cool surfaces (such as windows, floors, ceilings, and exterior walls). |
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COP
(Coefficient Of Performance) back to "Find the definition for . . ." |
. . . a measurement of a heat pump's efficiency (in the heating mode) at a specific outdoor temperature—usually 47°F. A COP of 1.0 indicates that for each unit of energy being used, an equal amount of energy (in the form of heat) is being provided by the system. A heat pump with a COP of 3.0 would provide three times as much energy in heat as it consumes in electricity when the outdoor temperature is 47°F. COP is also sometimes used to measure the single-temperature cooling efficiency of chillers. Learn more about COP. |
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Daylighting back to "Find the definition for . . ." |
. . . the technique of using natural light from windows, skylights and other structural openings to supplement or replace a building's artificial lighting system. When applied properly, daylighting can reduce a facility's lighting costs. When applied improperly, however, it can not only lead to inappropriate light levels but can also raise the building's cooling costs by introducing high levels of solar heat gain into the interior of the building. |
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Dedicated fixture back to "Find the definition for . . ." |
. . . a lighting apparatus that is designed specifically for use with a particular type of lamp (bulb). For example: the increasing popularity of CFLs has led to the development of a growing number of fixtures—including floor torchieres, table lamps, ceiling drums, and recessed canisters—dedicated solely for use with compact fluorescent lamps. |
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EER
(Energy Efficiency Ratio) back to "Find the definition for . . ." |
. . . a measurement of the energy required by a cooling system as it attempts to maintain indoor comfort when the outdoor temperature is at a specific temperature—usually 95°F. The term EER is most commonly used when referring to window air conditioners and geothermal heat pumps. EER equals the number of Btu-per-hour worth of cooling provided at the specified outdoor temperature divided by the number of watts used to provide that level of cooling. Learn more about EER. |
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Efficiency |
. . . the degree to which a certain action or level of work can be produced for the least expenditure of effort or fuel. For instance, a light bulb that uses 15 watts of electricity to produce 900 lumens of light would operate with much greater efficiency than one that required 60 watts to produce the same light level. |
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HSPF
(Heating Seasonal Performance Factor) back to "Find the definition for . . ." |
. . . a measurement of an all-electric air-source heat pump's efficiency (in the heating mode) over an entire season. HSPF is calculated by dividing the total number of Btus of heating provided over the entire season by the total number of watt-hours required to operate the system over the season. Learn more about HSPF. |
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Insulation back to "Find the definition for . . ." |
. . . a product that inhibits conductive and convective heat transfer. Some materials are naturally better insulators than others because they contain more "dead air" pockets. These pockets of trapped gas help to slow the movement of heat. However, if processed properly, virtually any product, including glass, cotton, paper, and plastic, can be used to make insulation. |
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Internal
heat gain back to "Find the definition for . . ." |
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. . the accumulation of heat produced by a building's energy systems,
appliances and occupants. Depending on the number of occupants and the
type and number of energy systems used during the day, it's not unusual
for internal heat gain to account for 20% of a home's total summer
cooling load. Internal heat gain can also help to reduce the need for mechanical heating in
winter. Solar gain and air infiltration also contribute to overall heat gain in a building. |
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Kilowatt (kW) back to "Find the definition for . . ." |
. . . 1000 watts. |
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Kilowatt-hour
(kWh) back to "Find the definition for . . ." |
. . . 1000 watts used for one hour—or any combination of energy multiplied by time that is equivalent to that rate of electrical consumption (such as one watt used for 1000 hours, 10 watts used for 100 hours, or 50 watts used for 20 hours). For example, a 100-watt light bulb left on for five hours a day would consume one kWh of electricity in two days. Kilowatt-hour is the primary measure on which U.S. electric companies base most customer billing. Most CWLP residential customers pay an average of 9.5¢ per kWh. (This figure, which includes the average per-kWh-rate paid by our regular residential customers, plus the fuel adjustment charge and Illinois State Utility Tax that are applicable to each kWh of use., was last updated on September 30, 2008.) |
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Low-E back to "Find the definition for . . ." |
. . . (which stands for low-emissivity) refers to a material designed to reduce the amount of radiant heat that can be transferred through glass or other translucent window coverings. Low-E coatings or films have the ability to re-radiate a high percentage of heat back toward its source. In summer, low-E windows can be effective in reducing the amount of solar gain in the building. In winter they can reduce the amount of furnace-generated heat that can be lost to the outdoors. |
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Lumen back to "Find the definition for . . ." |
. . . a unit of light given off by a light source. Lumen is the measurement used to compare the levels of illumination provided by different light sources. For instance, a 15-watt compact fluorescent lamp (CFL) will produce approximately the same number of lumens as a 60-watt incandescent bulb. |
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Payback period back to "Find the definition for . . ." |
. . . the amount of time it takes to achieve a full return on an investment. For instance, if a high-efficiency air conditioner would cost you $300 more to purchase than a lower-efficiency model but would save you $100 a year in operating costs, your payback period on the extra $300 investment would be three years. |
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Radiation back to "Find the definition for . . ." |
. . . a method of heat transfer by which heat is transmitted from surface to surface via infrared waves. Radiant heat warms the surfaces it touches without increasing the temperature of the air through which it travels. All warm bodies radiate infrared energy. |
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Rate back to "Find the definition for . . ." |
. . . the amount charged to utility customers per unit of service used. For instance, CWLP electric customers are charged a certain rate for each kilowatt-hour of electricity they consume (see Residential Electric Rates or Business Electric Rates), while water customers are charged per unit of water consumed (see Water Rates). The actual rate charged is based on the type of service provided and, in the case of water customers, whether or not the customer is located inside of outside the city limits. |
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Return on investment (ROI) back to "Find the definition for . . ." |
. . . the annual rate at which an investment earns income. It is calculated by dividing the annual earnings by the original investment. For instance, a bank savings account paying $3 per year per $100 investment, has an ROI of 3% ($3 divided by $100). An efficiency investment's ROI comes not from money paid to you, but rather from money saved by you on your energy bills. |
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R-value back to "Find the definition for . . ." |
. . . a measurement of a material's ability to resist heat transfer. Insulation products are rated according to their R-value. The higher the R-value, the better a product will be able to resist heat flow. Learn more about R-value. |
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SEER
(Seasonal Energy Efficiency Ratio) back to "Find the definition for . . ." |
. . . a measurement of the energy efficiency with which a central cooling system can operate over the course of an entire cooling season. This term is most often applied to central air-source heat pumps (in the cooling mode) and air conditioners. SEER is expressed as the dividend of the number of Btu of cooling provided over the season divided by the total number of watt-hours the system consumes. Federal law requires all central split systems now made and sold in the United States to have minimum SEERs of 13. Learn more about SEER. |
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Settled density back to "Find the definition for . . ." |
. . . the amount (depth) of insulation remaining after the insulation has had a chance to settle. This term is most often applied to loose-fill insulations—particularly those made of cellulose. To ensure loose-fill cellulose insulation will maintain its desired insulating value (R-value) once it has settled, you will need to install it to a depth that is 20% to 25% deeper than your settled density R-value actually calls for. |
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Solar gain back to "Find the definition for . . ." |
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. . heat that builds up inside a structure as a result of sunlight that
enters through transparent or translucent surfaces, such as windows, and
is converted to heat after striking other surfaces inside the building. In
summer, solar gain can cause as much as 50% of the
heat gain in a home. In summer, heat gain can increase the operating load
on your air conditioner. In winter, heat gain, can reduce the operating
load on your heating system. Internal heat gain and air infiltration also contribute to overall heat gain in your building. |
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Thermostat
setback back to "Find the definition for . . ." |
. . . an intentional effort to control building energy consumption by manually or automatically controlling thermostat settings according to the amount of cooling or heating that is needed at any given time of the day or night. |
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U-value back to "Find the definition for . . ." |
. . . the measurement of how readily heat can flow through glass, brick, drywall and other building materials. U-values, which are expressed in decimals, are the opposite of R-values. The higher the U-value, the less efficient the building material will be. Lean more about U-values. |
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Vapor barrier back to "Find the definition for . . ." |
. . . a material designed to resist the migration of moisture through a wall or other building component. As water vapor in the air moves from a warmer to a cooler part of the building it can condense on cooler building components, such as rafters, beams and walls, eventually causing those components to mildew, rust or rot. Vapor barriers, which are impermeable to water vapor migration, help to protect against this. The most common vapor barriers are made of plastic, but other materials, including oil paint, can also serve the purpose. |
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Watt back to "Find the definition for . . ." |
. . . a unit of electric power. The amount of power required by electric appliances is expressed in watts. One watt equals 1/1000 of a kilowatt. |
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Watt-hour back to "Find the definition for . . ." |
. . . a unit of electric energy, equal to one watt used over a period of one hour. One watt-hour equals 1/1000 of a kilowatt-hour. |
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Weatherstripping back to "Find the definition for . . ." |
. . . a product designed to seal the cracks that exist between two moving parts or one moving and one stationary part of windows, doors and other movable building components. Weatherstripping is used to improve a building's energy efficiency by preventing air infiltration. |
Energy Systems and Technologies
Last updated: 02/23/09