Skip links

Energy Glossary

Use our glossary to understand the technical jargon used in the energy industry.

Adjustable Speed Drive (ASD)
a control device that varies the shaft speed to the driven load on a motor. One example is the variable frequency drive (VFD), sometimes called a variable speed drive. A VFD consists of an electronic power converter that converts constant frequency AC (alternating current) power input into a variable frequency output. The AC motor speed varies in proportion to the drive output frequency.
AFUE (annual fuel utilization efficiency)
a measure of heating efficiency on an annual basis. AFUE is defined as the heat transferred to the conditioned space divided by the total fuel supplied (in Btu) for a given heating season. The higher the AFUE rating, the greater the efficiency.
the basic unit of electric current, one ampere represents the rate of one coulomb of charge per second. It is defined to be the total net charge flowing across a given cross-sectional area of wire per second.
all fluorescent and most metal halide lamps need ballasts to operate. Because the arc generated in a lamp has almost no electrical resistance, ballasts are provided to regulate the lamp current. Magnetic ballasts consist of a coil in series with the lamp, or may also include a capacitor to improve power factor. Electronic ballasts are more efficient and regulate lamp current by delivering high frequency pulses. Electronic ballasts do not have the large coils and associated heat loss of magnetic ballasts. They not only operate more efficiently themselves, but electronic ballasts can also improve the efficacy of the lamp-ballast system by 10%-20%.
Ballast Factor
the ballast factor is the ratio of the light output of a fluorescent lamp to the light output of the same lamp operated on a standard (reference) ballast. The ballast factor is multiplied by the rated lumens to estimate how much light actually comes from the tube. T8 electronic ballasts may have a typical ballast factor of 0.88 to 0.95, but can range from 0.6 to 1.3.
BAS (building automation system)
optimizes the performance of HVAC equipment and alarm systems by automating the interaction of building mechanical subsystems, improving occupant comfort, lowering energy use and allowing off-site building control.
a type of space heating equipment consisting of a vessel or tank where heat produced from the combustion of fuels such as natural gas, fuel oil, or coal is used to generate steam or hot water. Unlike a furnace, air is not forced through the structure; the steam or hot water is circulated through pipes to radiators or baseboards where it warms the surrounding air by convection. The two most common boiler layouts are firetube and watertube.
Boiler Horsepower (bhp)
rate of water evaporation equal to the evaporation per hour of 34.5 pounds of water at a temperature of 100°C (212°F) into steam at 100°C. One boiler horsepower is about 33,475 Btu per hour (about 9809.5 watts).
a British Thermal Unit is the amount of heat required to raise one pound of water by one degree Fahrenheit. Approximately equivalent to the thermal energy of a kitchen match.
an SI (International System of Units) unit of luminous intensity. One candela is one lumen per steradian, where a steradian is a unit of measure relating the angle subtended at the center of a sphere to the sphere's surface area and radius.
an electrical energy storage device, consisting in general of two metallic plates separated and insulated from each other by a dielectric.
a common billing term that refers to 100 cubic feet of natural gas.
CFC (Chlorofluorocarbon)
a compound consisting of chlorine, fluorine, and carbon. CFCs are very stable in the troposphere. They move to the stratosphere and are broken down by strong ultraviolet light, where they release chlorine atoms that then deplete the ozone layer. CFCs are commonly used as refrigerants, solvents, and foam blowing agents. The most common CFCs are CFC-11, CFC-12, CFC-113, CFC-114, and CFC-115.
a cooling system that uses mechanical vapor compression (centrifugal, screw, scroll, or reciprocating) or absorption to achieve refrigerant phase change. All chillers have evaporators and condensers, though absorption systems use thermal energy to boil a solution to liberate the refrigerant instead of the electrical energy used in a vapor compression chiller. Chillers are typically found in medium to large commercial and industrial facilities, and can range from 25 to 1,500 refrigerant tons.
Coefficient of Performance (COP)
ratings that are more typically found in chiller ratings and gas cooling equipment, as well as in heat pumps. COP is a measure of how efficiently a heating or cooling system will operate at a single outdoor temperature condition. As an example, the commonly used outdoor temperature condition for a heat pump calculation in the heating mode is 47ºF. As is also the case for EER and SEER, a higher COP means higher efficiency.
the combined production of power and useful heat by the sequential or simultaneous use of energy from one fuel input into a subsequent process. As an example, steam that is generated for a manufacturing process can be used to generate electricity before going to the manufacturing unit. Widely used in businesses found in the pulp and paper industry, petrochemical, metals, mining, and food industries, it is also used on a smaller scale in commercial buildings such as hotels, hospitals, universities, and shopping centers.
Coloring Rendering Index (CRI)
a measurement of a light source's ability to render colors the same as sunlight does. Generally speaking, the CRI indicates the ability of a lamp to allow individual colors to be seen and to distinguish between these colors. On a scale of one to one hundred, light sources should have a minimum color-rendering index (CRI) of eighty for most interior spaces. Ceramic metal halide lamps, the latest "second generation" T8 lamps, T5 lamps, and most compact fluorescent lamps have a CRI in the range of eighty-two to eighty-six. Incandescent lamps have a CRI near one hundred, because they emit all the colors within the visible color spectrum.
Combined Heat and Power (CHP)
also referred to as cogeneration because it includes the simultaneous production of electrical and thermal energy.
Condensing Boiler
a highly efficient boiler that is able to recover waste heat by cooling some of the exhaust gases, and then capture the thermal energy of the condensate through a heat exchanger. Condensing furnaces and boilers require stainless steel or aluminum materials in their heat exchangers to protect them against the mildly acidic and corrosive condensate. Specialty plastic tubing is also commonly used to return the condensate within the heating system.
Cooling Degree Day (CDD)
the mean daily temperature minus 65°F.
Correlated Color Temperature (CCT)
the apparent/perceived color of a light source compared to the color appearance of an ideal incandescent light source at a particular temperature, expressed in degrees Kelvin (K). The CCT rating is an indication of the warmth or coolness of the light, with ratings below 3,200°K referred to as warm, and ratings above 4,000°K considered cool in appearance. As CCT increases, the appearance shifts from reddish white toward bluish white. Cool light is preferred for visual tasks because it produces higher contrast than warm light. The color temperature of a lamp has nothing to do with how hot the lamp will get or how much heat will be given off by the lamp.
Cubic Foot (cf)
as it relates to energy, one cubic foot of natural gas at standard temperature and pressure (60°F, 14.7 psi) contains approximately 1,030 Btu. Other energy billing units include MCF (1,000 cf, or approximately 1,030,000 Btu).
the use of daylight as a primary source of illumination in a space. It has become a more important feature of mainstream construction due to its ability to lower energy costs.
Demand (kW demand)
the maximum number of kilowatt-hours per defined time interval used by the customer. This is typically based upon the largest number of kilowatt-hours used in any half-hour or fifteen-minute period of the billing period. Demand Charges, a charge per kW or kVA of monthly billing demand, reflect the electric utility's infrastructure cost of power generation and transmission as well as the more expensive fuels used in peaking plants to meet each customer's highest monthly power peak.
Distributed Generation (DG)
a modular electricity generation or storage concept that involves locating smaller generators, based on a variety of technologies, closer to the electrical loads that will be using them. DG has the potential to serve as a reliable source of standby or emergency backup power and, when used with uninterruptible power supplies, can offer improved reliability for interruption-sensitive equipment.
EER (energy efficiency ratio)
a measure of how efficiently a cooling system will operate when the outdoor temperature is at a specific level (usually 95°F). A higher EER means the system is more efficient. EER = Btu of Cooling @ 95°F / Watts used @ 95°F. In the case of a 10 EER, 2 ton air conditioner: 10 EER = 24,000 Btu Out / 2,400 Watts In.
the ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt (LPW). As an example, an incandescent bulb may provide 1,700 lumens from a 100 watt light bulb, providing an efficacy of seventeen lumens per watt. Fluorescent lamps have efficacies from thirty to ninety-five lumens per watt, and high intensity discharge lamps (metal halide, high pressure sodium, etc.) can have efficacies exceeding 150 lumens per watt.
Electrical Ground
an electrical means to intentionally create a low-resistance path that connects to the earth. This enhances safety and equipment protection by preventing the buildup of voltages that could cause an electrical accident or shocking hazard. Proper grounding practices are specified by the National Electric Code, IEEE standards, and building codes to insure uniformity and safety in electrical systems.
EMS (energy management system)
reduces energy use in buildings by monitoring conditions and controlling energy consuming equipment. An EMS is typically applied to the largest electrical loads, including HVAC equipment, cooling towers, pumps, water heaters, and lighting.
a glass tube with electrodes at each end and an inside surface coated with one or more types of phosphors. Mercury vapor fills the tube at very low pressure, along with one or more "buffer" gases. Fluorescent lamps from least efficient to most efficient include the T12, T8, and T5, where the number represents the diameter of the tube in 1/8" increments (e.g., a T12 is twelve eighths, or 1-1/2" diameter). As the diameter gets smaller, glare increases. There are also high output T8 and T5 lamps that produce greater lumen output.
the distribution of light on a horizontal surface is called its illumination, which is measured in foot-candles. A foot-candle of illumination is a lumen of light distributed over a one-square-foot (0.09-square-meter) area. Full sunlight produces an illuminance of about 10,000 foot-candles on a horizontal surface. Full moonlight provides an illuminance of about 0.02 foot-candles. Adequate illumination for reading is taken to be about ten foot-candles; machine shop work requires about forty foot-candles. One foot-candle equals 10.76 lux.
a type of space heating equipment with an enclosed chamber where fuel is burned or electrical resistance is used to heat air directly, without using steam or water. The heated air is then blown or forced throughout the building, typically via air ducts, where it enters a room through a register in the floor or the wall to provide heat via convection. Furnaces can be powered by a number of different means, such as electric, gas, fuel oil, coal, or wood.
Halogen Incandescent
also called tungsten halogen lamps, the filament is surrounded by a hot gas from the halogen family (fluorine, chlorine, bromine, or iodine) to improve efficiency. The halogen gases allow for regeneration of the tungsten filament, thus enabling the filament temperature to be increased for higher efficiency. These lamps are primarily used commercially in theater, store, and outdoor lighting systems.
non-linear signals inherent in electrical systems that have motors, variable speed drives, capacitors, inductive devices or other electrical circuitry not operating in a purely resistive or linear mode.
HCFC (Hydrochlorofluorocarbon)
a compound consisting of hydrogen, chlorine, fluorine, and carbon. Developed as an interim bridge between CFC and HFC refrigerants, HCFC production and consumption is currently capped in the United States. As of 2020, HCFC-123 can no longer be used in new equipment and production will cease in 2030. HCFC-142b and HCFC-22 will be phased out of new equipment by 2010. According to the EPA, HCFCs contain chlorine and thus deplete stratospheric ozone, but to a much lesser extent than CFCs.
Heat Pump
takes advantage of underground thermal energy or outside air to provide space conditioning heating, cooling, and humidity control. Rather than converting chemical energy to heat, as in a furnace, a heat pump works by moving heat. Heat pumps pull heat from the air or earth to provide heat in the winter. In the summer, the heat pump becomes an air conditioner that pulls heat from a building by transferring it to the earth or the air. Heat pumps can be air source or water source (geothermal), and can also be found in industrial applications.
Heating Degree Day (HDD)
65°F minus the mean daily temperature.
Heating Seasonal Performance Factor (HSPF)
the ratio of the seasonal heating output in Btu divided by the seasonal power consumption in watts for a heat pump. Like SEER ratings for air conditioners, HSPF is a seasonal measure. HSPF ratings of seven to nine are common for air source heat pumps, while geothermal (water source) heat pumps typically have HSPF ratings greater than ten. As an example, an HSPF of 8.5 correlates to 249% efficiency (8.5 / 3.412 x 100).
Henry Hub
a popular pricing point for natural gas spot and futures contracts that is traded on the NYMEX and other exchanges, it is physically located in Louisiana at the intersection of nine interstate and four intrastate natural gas pipelines.
HFC (Hydrofluorocarbon)
a compound consisting of hydrogen, fluorine, and carbon developed as a class of replacements for CFCs. Because they do not contain chlorine or bromine, they do not deplete the ozone layer. All HFCs have an ozone depletion potential of zero.
High-Intensity Discharge (HID)
an electric lamp that produces light directly from an arc discharge under high pressure, such as mercury vapor, metal halide, and high and low-pressure sodium. There are large differences in efficiency, color rendering, and other characteristics within HID lamps. HID lights require starting times of between two and fifteen minutes before reaching full output.
High-Pressure Sodium (HPS)
these lamps use sodium as the main light-emitting element, though these lamps also contain some mercury. Most HPS lamps require an igniter, in addition to a ballast. The igniter generates high voltage pulses for starting. The ballast regulates the current after the lamp has been started. Efficacies can vary from 50 to 150 lumens per watt, and CRIs between twenty and eighty-five. Lighting color is yellowish white, with the disadvantage that the lower CRI yellowish light can make it difficult to distinguish the colors of objects illuminated by it.
Horsepower (hp)
a unit of power, typically ascribed to motors, equivalent to working at the rate of 33,000 ft-lb/minute, or 550 ft-lb/sec. One hp = 746 watts. Can be equated to electrical power in motors, kW = 0.746 hp / motor efficiency.
light is emitted by a hot glowing body, commonly a tungsten filament. Incandescent lighting is characterized by very high CRI, but very low efficiency.
Infrared (IR)
electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation, the name means "below red" (from the Latin infra, "below"). Energy is transmitted in wavelengths from an infrared emitter directly to the object. Infrared is typically divided into three wavelength categories - short, medium, and long - that fall between 700 nm and 1 mm. An alternative to convection heating, infrared is used in drying and curing for many industrial applications, including pulp and paper processing, paint drying and curing, plastics, and textile manufacturing. Fuel source can be either electric or gas.
Initial Light Output
a lamp's output, in lumens, after 100 hours of seasoning.
Instant-start Ballast
this type of ballast applies a high-voltage across the electrodes, up to twice as high as with other starting methods, to strike the electric arc. The higher voltage is necessary because the electrodes are not heated prior to starting. It is 5%-10% more energy efficient than rapid or preheat starting, but causes greater wear on the electrodes during starting. Newer, improved designs minimize the degradation of life caused by frequent switching, though it may still be an issue for some types.
Kilovolt-Ampere (KVA)
a unit of electrical power equal to 1,000 volt-amperes.
Kilovolt-Ampere Reactive (KVAR)
the electric utility must supply more power to account for the losses incurred due to excessive inductance or capacitance. If the useful power that the energy user consumes is described as "Working Power" or "Real Power" (kW), then the component of lost power is sometimes referred to as Reactive Power, or kilovolt amps reactive (KVAR).
Kilowatt Demand (kW demand)
the maximum number of kilowatt-hours per defined time interval used by the customer. This is typically based upon the largest number of kilowatt-hours used in any half-hour or fifteen-minute period of the billing period. Demand Charges, a charge per kW or kVA of monthly billing demand, reflect the electric utility's infrastructure cost of power generation and transmission, as well as the more expensive fuels used in peaking plants to meet each customer's highest monthly power peak.
Kilowatt-Hours (kWh)
the quantity of true power multiplied by time. 1,000 watts of power used for one-hour equals one kilowatt-hour. Kilowatt-hours are referred to as true power because they are a measure of that portion of the kilovolt-ampere hours that can be converted from electrical energy into some other form of useful energy, such as heat, light or motion.
the eye perceives visible light over a range from 380 nanometers (nm) to 780 nm, with a range of 400 to 700 nm quite common. Human eyes typically have a maximum sensitivity of ~ 555 nm (in the green spectrum). The eye may, however, have some visual response at even wider wavelength ranges.
Load Factor
the ratio of the average electrical load divided by peak load during a designated period of time. As an example, if a business used 10,000 kWh during one month (720 hours) and had a maximum demand of 25 kW, then the load factor would be 56 percent (10,000 kWh x 100)/(720 hours x 25 kW).
Low-Pressure Sodium (LPS)
similar to high-pressure sodium lamps, except that the efficacy is higher at the expense of a lower CRI. Some commercial LPS lamps have achieved an efficacy of 180 lumens per watt, but their CRI is very poor, ranging from zero to twenty. Low-pressure sodium bulbs are monochromatic, making it difficult to distinguish color under these lights. Starting times of between seven and fifteen minutes are also required before they reach full output. Typical applications include highway and security lighting.
the lumen is a unit of luminous flux, as defined in terms of the candela, which is the foundation unit for the measurement of visible light.
Lumen Maintenance
a measure of how well a lamp maintains its light output over time. It is the rated mean light output as a percentage of rated initial light output. It may be expressed numerically or as a graph of light output vs. time. Numerical lumen maintenance numbers are usually a percentage of initial light output at 40% of rated life.
a measure of illuminance in lumens per square meter. One lux equals 0.093 foot-candles.
Medium Bi-Pin
a type of connector commonly used on T8 and T12 fluorescent lamps. Two small pins protrude from the lamp ends, which are inserted into a socket in the fixture. T5 fluorescent lamps use a similar but smaller connector called a miniature Bi-Pin, which is not interchangeable with a medium bi-pin.
this substance has many desirable optical properties, making it the primary light-emitting vapor used in fluorescent lighting. It is also commonly used in high intensity discharge lighting. Light is emitted when mercury atoms relax from their first excited energy level at a wavelength of 254 nanometers. Phosphors convert this UV light into visible light, and buffer gases (argon) can be added to further improve lighting quality.
Mercury Vapor (MV)
the least efficient of the HID lamp types, almost all of their light is emitted by mercury vapors. CRI is on the low end, typically in the range of forty to fifty-five. MV lamps compete with high-pressure sodium and metal halide for outdoor lighting applications where lighting quality is less important. MV lamps have the longest life of HID-type lamps, but are not very efficient - with efficacies ranging from 35 to 65 lumens per watt. Lighting color is in the bluish white spectrum.
Metal Halide (MH)
evolved from mercury vapor lamps, MH lamps use a variety of metal vapors, rather than mercury vapor alone, to achieve better lighting quality (CRI ratings from sixty to ninety). Metal halide implies that the additional metals include compounds with some of the halogen elements (fluorine, chlorine, bromine, or iodine). MH lamps are not interchangeable with mercury vapor lamps, except in some small sizes. Efficacies range from 75 to 125 lumens per watt.
a small-scale power generation source approximately the size of a refrigerator, ranging typically from 25 to 200 kW. Advantages include: small number of moving parts, compact size, light-weight, greater efficiency, lower emissions, lower electricity costs, and opportunities to utilize waste fuels. Microturbine systems are comprised of a compressor, combustor, power turbine, generator, and often a recuperator.
Packaged Rooftop HVAC
also referred to as packaged heating or packaged cooling equipment, these systems contain the compressor, evaporator coil, condenser, heating unit, supply fan, exhaust fan, and filter in one preassembled package that can be installed as a self-contained unit. A number of sizes and designs are available and most are commonly found on the rooftops of office buildings, retail stores, malls, schools, manufacturing plants, and a host of other commercial and industrial facilities. Can be fueled by gas, electric, fuel oil or a combination of these fuels.
used in fluorescent and HID lamps to improve lighting quality, phosphors are crystals that convert the UV light emitted by the mercury vapor into visible light. The makeup of the various phosphors determines the color emitted by the fluorescent or HID light.
Power Factor (PF)
a ratio equal to the real power (kW) divided by the apparent power (kVA). PF = kW/kVA
Radiant Heating
relies on the concept of radiant heat transfer, in which heat produced by the system radiates to nearby objects and surfaces. Just as the sun heats people and objects even when the air is cold, radiant heating can warm the people and objects in a room without having to warm the air in the room. The most common form of radiant heater is an infrared heater.
Rapid-Start Ballast
this type of ballast utilizes a method of starting fluorescent lamps in which lamp electrodes are heated prior to starting, using a starter that is an integral part of the ballast. Heating the electrodes before starting the lamps reduces the voltage required to strike the electric arc between the electrodes. A rapid-start system starts smoothly, without flashing.
Reactive Power (kVAR)
the quantity of non-working power caused by magnetizing current. Reactive power is measured in kilovars or kVARs. Reactive power is required for certain devices to operate, but it is not available as useful power on the output side of the device. Examples of such devices are motors, transformers, relays, and fluorescent lights.
a compound used in a heat cycle that undergoes a phase change from a gas to a liquid and back. The main uses of refrigerants are in chillers, air conditioners, and refrigerators/freezers.
Relative Humidity (RH)
a measure of the amount of water in the air compared to the amount of water the air can hold at a particular temperature. Warmer air has more capacity to "hold" water vapor than colder air. Dew point is a measure of how much water vapor is actually in the air.
SEER (seasonal energy efficiency ratio)
a measure of efficiency over an entire cooling season, as opposed to a single outdoor temperature. Residential units are almost always rated in SEER. SEER came into use as a more practical measure, since the temperature outside is not always 95ºF. Also, the denominator is in watt-hours, not in watts as is the case for EER. The same relationship holds true, i.e., a higher SEER means the system is more efficient. SEER is the total amount of cooling the air conditioner will provide over the entire cooling season divided by the total number of watt-hours it will consume, or SEER = Seasonal Btus of cooling / Seasonal watt-hours used.
Spark Spread
the difference between the price of electricity sold by a generator and the price of the fuel used to generate it, adjusted for equivalent units. Sometimes it is referred to as the price of electricity without the fuel component. Spark spread is often used as a decision factor to determine when to run a generator, by calculating the Btu equivalent generation cost for natural gas and electricity based on specific market conditions.
Standby Generator
supplies backup power to energy users in the event of a power outage. Typically limited to applications that require backup power to an existing power utility grid, standby generators are not designed to run as prime power or in a continuous application. They can play a strategic role in minimizing power costs and maximizing power reliability by ensuring uninterruptible, onsite standby capability in the event of power outages, both unexpected or planned.
Steam Trap
automatic valves found on the steam distribution lines of a boiler heating system to keep the system operating optimally by purging condensate and maintaining steam quality. The devices typically have a float to trap the condensate. Once the steam has condensed to become hot water, it is removed by the trap and is either returned to the boiler or discharged to the atmosphere.
an energy unit equal to 100,000 Btus. A decatherm is equal to 10 therms.
infrared thermography has been used in predictive maintenance and condition monitoring of electrical and mechanical equipment, as well as building performance testing. Infrared imagers capture infrared radiation emitted from objects and display these invisible wavelengths as visible light images. They are useful in applications where a variation in temperature, reflection, surface condition, or material may cause a difference in the radiated energy level detected by the equipment.
Three-Phase Power (3 phase)
a very efficient means of transmitting electrical energy whereby the sequential peaks (of phases A, B, and C) are spaced 120° apart to enable a relatively smooth power curve. The goal of efficiently delivering electrical power over a three-phase system is to either keep the phases balanced with the same amount of load or in symmetry with each other. Motors greater than 10 hp are almost always three-phase power, because they tend to be more efficient than single-phase motors.
Ton (refrigerant ton, RT)
defined as the cooling capacity of an air conditioning system. One ton is equal to the Btu thermal content required to melt one ton of ice in a 24-hour period. A 1-ton air conditioner is rated at 12,000 Btu, a 2-ton unit at 24,000 Btu, a 3-ton unit at 36,000 Btu, and so forth. It takes 144 Btu of heat to melt 1 pound of ice in 24 hours, or 288,000 Btu to melt a ton (2,000 pounds) in 24 hours. Typical residential central heating systems provide from 1 to 5 tons of cooling. Commercial rooftop units are typically 3 to 20 tons each. Chillers can range from 15 up to 1,500 tons.
Ultraviolet (UV)
electromagnetic radiation of a wavelength shorter than that of visible light, the name means "beyond violet" (from Latin ultra, "beyond"), violet being the color of the shortest wavelength of visible light. UV is also called black light, as it is invisible to the human eye. UV wavelengths are often subdivided into UV-A (380-315 nm), UV-B (315-280 nm), and UV-C (280-10 nm).
UPS (Uninterruptible Power Supply)
provides backup power in the event of a power outage. A UPS can range from a 300 VA unit to protect a single PC up to 1,000 kVA system to protect clusters of equipment or entire facilities, such as those found in telecommunications, financial services, hospitals, and industrial sites with mission critical processes. In addition to providing backup power, UPS systems can also provide "conditioned" power. The three types of UPS systems include off-line, line-interactive, and on-line.
the basic unit of voltage, or electric potential difference. It is the difference of electrical potential between two points of an electronic circuit. One helpful description uses a water analogy, equating voltage (volts) to the fluid pressure in a water pipe system, and current (amps) to the actual flow of water.
a measure of power that is the product of voltage (volts) and current (amps). It is roughly equivalent to 3.412 Btu/hr. One kilowatt (kW) equals 1,000 watts.