Glossary of Energy Terms

AHAM: Association of Home Appliance Manufacturers

ARI: Air-Conditioning and Refrigeration Institute

ASHRAE: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.

Btu: British Thermal Unit

Capacity (Cooling): The quantity of heat in Btu which an air conditioner or heat pump is capable of removing from an enclosed space in one hour, expressed as thousands of Btu/hr, millions of Btu/hour (MBtu/hr), or tons. A unit rated at one ton can remove 12,000 Btu/hr of heat.

Cooling degree hours: A term useful in expressing the severity of the weather in a region during the cooling season. The cooling degree hours in a season are derived by summing the difference between the average outdoor temperatures above a base (e.g., 74 degrees F) each hour and the base temperature. Statistics gathered over recent years suggest a growing number of cooling degree hours in many regions, perhaps owing to the greenhouse effect. Cooling degree hours are used in computing seasonal energy flows in a building due to both conduction and convection.

Coefficient of performance (COP): The ratio of energy delivered by an air conditioner or a heat pump to the input energy, where both forms of energy are measured in the same units (typically Btus).

Economizer System: During the cooling season, whenever outside air temperatures go below inside set point temperatures, it is usually appropriate to terminate the use of compressor-based cooling and open HVAC system dampers to supply 100% outside air. This economizer strategy is frequently cost effective in southwestern climates where the absence of clouds during the cooling season allows outside temperatures to drop quite substantially in the evening. Economizers must be maintained and controlled with care to ensure that dampers are fully opened when needed—and fully shut when not.

Energy-Efficiency Ratio (EER): The ratio of the cooling capacity in Btus to the electric input to the air conditioner or heat pump in watts.

Gray water system: Device and associated fixtures through which energy associated with the heat from warm water that goes down drains can be recovered using water-to-water or water-to-air heat exchangers.

Heating degree days: A term useful in expressing the severity of the weather in a region during the heating season. The heating degree days in a season are derived by summing the difference between the average outdoor temperatures above a base (e.g., 65 degrees F) each 24 hours and the base temperature. Heating degree hours (equal to heating degree days x 24) are used in computing seasonal energy flows in a building due to both conduction and convection.

Heating Seasonal Performance Factor (HSPF): The total heating output of a central air conditioning heat pump, in Btu, during its normal usage period for heating, divided by the total electric energy input in watt-hours during the same period.

Natural air changes per hour (nach): Buildings have openings in their conditioned envelopes which cause the exchange of exterior air with inside air. The rate at which this exchange occurs is a function of many variables, including the magnitude, shape, and locus of the openings, the temperature difference between inside and outside of the envelope, the height of the building, and other factors which cause pressure differences. Instantaneous air exchange rates are difficult to measure, as are averages over a heating or cooling season, but a good estimate of nach is critical in estimating convective energy flows.

Phase: Phase of electrical service—single-phase or three-phase. Three-phase electrical service is routine in industrial settings and large horsepower motors are routinely wired for three-phase service.

Seasonal Energy-Efficiency Ratio (SEER): The ratio of the total cooling capacity in Btu during a normal annual usage period to the total electric input in watt-hours to the air conditioner or heat pump during the same period.

Shading coefficient (SC): The shading coefficient of a window is the ratio of total solar transmittance to the transmittance through 1/8-inch clear glass. SC, which is being phased out as a glazing metric, is approximately equal to 1.15 times the solar heat gain coefficient.

Single-Package System: An air conditioner or heat pump with all major components housed in a single casing.

Solar heat gain coefficient (SHGC): SHGC is the fraction of the incident solar energy transmitted through a window. Windows with low SHGC values improve comfort for building occupants near sunlit windows, lower the total cooling load of the building, and help smooth out the difference in cooling loads between perimeter and core zones.

Split System: An air conditioner or heat pump consisting of two or more major components which are not enclosed in one cabinet; for a split system, a compressor and condenser are generally installed outside the building and the cooling coil is generally installed within the building.

Temperature, dry bulb and wet bulb: The temperature of air measured with a thermometer whose sensing element is dry is known as “dry bulb temperature.” If a thermometer’s sensing element is surrounded by a wet wick over which air is blown, it reads “wet bulb temperature.” When the relative humidity is at 100%, there is no difference between dry and wet bulb temperatures, but as the relative humidity of the air descends, so does the wet bulb temperature with respect to dry bulb temperature. In climates such as those in the southwest, where humidity is frequently quite low, the differences are substantial. For example, at 10 percent relative humidity and a dry bulb temperature of 90ºF, the wet bulb temperature is 58ºF, a 32 degree difference. Such large differences between dry and wet bulb temperatures favor evaporative cooling techniques.

UA: The heat transfer coefficient times cross sectional area. U-factor is the heat transfer coefficient of a material or set of materials that make up the space between the inside and outside of a conditioned space. It is measured in Btu’s per hour per square foot per degree Fahrenheit (Btu/h-ft²-ºF). It is the inverse of R-value which expresses resistance to conductive heat transfer. A is the cross sectional area of the surface in question and has units of square feet. Accordingly, the product, UA, has units of Btu/h-ºF. Thus, multiplying UA times the temperature difference between inside and outside of the conditioned space yields an expression of energy flow in Btu/hr.

Variable air volume (VAV): An HVAC system strategy through which the volume of air delivered to conditioned spaces is varied as a function of ventilating needs, energy needs, or both.

Variable frequency drive (VFD): A common type of variable speed drive which controls motor speed by changing the frequency of the voltage supplied to the motor.

Variable speed drive (VSD): An electronic device between the voltage source and a motor or a mechanical device between a motor and its load that enables the controlling of the speed of a motor.

Visible transmittance (Tv): Tv is the percentage of visible light that passes through a window.

The basic unit of heat energy is the British Thermal Unit, Btu, the amount of heat necessary to raise a pound of water a degree Fahrenheit.

A therm has the energy content of 100,000 Btu. A million Btu is written MBtu and is the energy equivalent of about a person year of labor.

Electric power is measured in watts, where a watt = 3.412 Btu

Electric energy is measured in watt hours. It is also expressed in kilowatt hours (kWh) = a thousand watt hours, megawatt hours (mWh) = a million watt hours, gigawatt hours (gWh) = a billion watt hours, and terrawatt hours (tWh) = a trillion watt hours.

The table below expresses some relationships between forms of energy:

Light levels are measured in lumens, defined as the light flux falling on a surface one square foot in area, every part of which is one foot from a point source having a luminous intensity of one candela in all directions. A candela is the international basic unit of light from which all others are defined; it’s a primitive term, like mass, time, and distance in Newtonian mechanics. An ordinary wax candle has a luminous intensity in a horizontal direction of approximately one candela. The efficiency of a light source is called luminous efficacy and has the units of lumens per watt. The chart below gives the luminous efficacy of various light sources:

In the case of electric lighting, the “watts” part of lumens/watt usually comes from the local utility company. For the sun, watts are associated with the portion of heat per unit of light that accompanies natural light. Direct beam is about 113 lm/W and a northern sky about 180 lm/W. Accordingly, using natural light to replace electric light saves energy two ways; the light is free and the air conditioning bill is lowered.

A short ton is defined as 2000 pounds; a metric ton is 1000 kilograms = 2204 pounds.

CO₂, carbon dioxide, is the most predominant greenhouse gas

NOx, oxides of nitrogen, are produced by the burning of fossil fuels