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Common Units and Conversions Energy use in the United States involves many diverse industries and sectors, each of which uses its own conventions and units to describe energy production and use. Although these units are in common usage throughout the energy industry, they are not always consistent and are not well un- derstood by nonexperts. Similarly, different types of units are employed to describe emissions resulting from energy-related use activities. This appen- dix describes the units used for principal energy supply and consumption activities and provides some useful conversion factors. The U.S. Department of Energy’s Energy Information Administration website provides additional information about energy (see www.eia.doe.gov/basics/conversion_basics. html) units and conversion factors, including easy-to-use energy conversion calculators. Total U.S. energy use in 2007 was 101.5 quadrillion (1015) Btu or 96 Exa (1018) Joules. Electricity • Electrical generating capacity is power and expressed in units of kilowatts (kW), megawatts (MW = 103 kW), and gigawatts (GW = 106 kW). It is defined as the maximum electrical output that can be supplied by a generating facility operating at ambient conditions. Coal power plants typically have generation capacities of about 500 MW; nuclear plants about 1,000 MW (1 GW); intermittent sources (e.g., natural gas peaking plants and individual wind turbines) about one to a few megawatts; and residen- tial roof-top installations of solar photovoltaics about a few kilowatts. • Electricity supply and consumption is expressed in units of kilowatt 40

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406 HIDDEN COSTS OF ENERGY hours (kWh), megawatt hours (MWh), gigawatt hours (GWh), or terawatt hours (TWh) (109 kWh). One kWh is equal to a power of 1,000 watts (the typical electricity that is consumed by a hand-held hair dryer) supplied or consumed over the period of an hour. Annual total delivered electricity in the United States is about 4,000 TWh and the average annual electricity consumption per U.S. household is about 11,000 kWh. o 1 kWh of electricity is equivalent to 3,410 Btu of thermal energy if the conversion has no inefficiencies. o In a 33% efficient power plant, 10,230 Btu of input primary energy are required to produce 1 kWh of electricity. Fossil Fuels and Other Liquid Fuels • Coal supply and consumption in the United States is usually ex- pressed in units of metric tons (sometimes written as tonnes and equal to 1,000 kg or 2,200 pounds [lb]) or short tons (2,000 lb); most of the rest of the world uses metric tons. This report uses short tons when discussing coal use in the United States. o A ton of typical coal contains about 22 MJ of energy. o A tonne of typical coal contains about 24 MJ of energy. • Petroleum and gasoline supply and consumption quantities are expressed in the United States in gallons or barrels (1 barrel = 42 gallons) and internationally in liters (3.88 liters = 1 gallon). In the United States, the energy content of liquid fuel is expressed in British thermal units (Btu), million Btu (MMBtu or 106 Btu), and quadrillion Btu (quad = 1015 Btu). The rest of world uses joules (J) to express the energy content of liquid fuels (1 Btu = 1,055 J). A Btu is defined as the amount of energy (in the form of heat) needed to raise the temperature of 1 lb of water by 1 degree Fahrenheit.1 The energy content of different fuels can be converted to Btu using the following approximate factors: o 1 barrel crude oil = 5,800,000 Btu = 5.8 MMBtu o 1 barrel gasoline = 5.2 MMBtu o 1 barrel fuel ethanol = 3.5 MMBtu • When different liquid fuels and blends are compared, this is often done on the basis of what volume would give the same energy as a gallon of gasoline. Therefore, about 1.5 gallons of ethanol would provide the energy equivalent of 1 gallon of gasoline. • Natural gas supply and consumption usage is expressed in units of a thousand cubic feet (MCF or mcf). This is the equivalent volume of gas at atmospheric pressure and temperature. Here the prefix M stands for 1A joule is the amount of energy needed to heat a kilogram of water by 1 degree centigrade. 1,055 joules = 1 Btu.

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40 COMMON UNITS AND CONVERSIONS a thousand, and MM is used to denote a million cubic feet. One MCF of natural gas contains about a million Btu of thermal energy. Basis for Quantifying Impacts • Activity-specific impacts result from particular energy use. For ex- ample, impacts from the emissions from an electric power plant or impacts from tailpipe emissions from a passenger car. • Activity-aggregate impacts are used to describe the impacts from energy use in a set of activities that include all impacts starting with the processing of primary energy, its conversions and its transportation to its end use point, its use to provide a set of energy services, and impacts as- sociated with disposal of end use equipment. The aggregations are based on life-cycle assessment (LCA) methods and use a variety of data and models to estimate the impact. For example, electricity use to provide light in a building would include all the “upstream inputs” to produce feed energy for the power plant (mining, dams, etc.), the electricity production inputs to generate and distribute power to the site of the light bulb, and impacts associated with operation of the light bulb. Waste heat from the bulb and its disposal would be “downstream impacts.” Larger downstream impacts would be associated with the health and other consequences from emissions at the power plant. In this report, life-cycle impact assessment (LCIA) is a goal that can only be achieved incompletely due to limitations in data availability and complexity of the detailed systems, but where important impacts are pres- ent their magnitudes are estimated to the extent possible. Waste Streams and Hazardous Air Emissions • Solid and liquid wastes are usually described using familiar units of volume or weight per unit time or quantity of energy produced. (cubic feet per minute [cfm]; tons per MWh; gallons per day; etc.). Where these waste streams contain contaminants, the concentration of the contaminant of con- cern is also important. (parts per million [ppm] by weight is the weight of contaminant in a million units of carrier weight; or pounds of contaminant per ton of carrier, or pounds of contaminant per gallon of liquid,) • Air emissions are usually described by emissions per unit of en- ergy produced or used—such as lb per MWh of electricity, lb per MCF of natural gas, or grams per vehicle miles traveled (VMT) and sometimes in terms of concentration of pollutants in emissions stream—such as parts per million (by volume) or pounds per cubic foot. The choice of a VMT basis is a compromise, since the more meaningful metric of passenger miles

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408 HIDDEN COSTS OF ENERGY traveled would require information about the number of passengers per vehicle—and would only change the final result if more passengers on average travelled on vehicles powered by a particular fuel. Presentation of results per gallon of fuel makes for difficult comparisons since different fuels have different energy contents per gallon. In this report, impacts are assessed nationally using detailed models for the overall activities. Using a VMT basis for the transportation emis- sions estimates includes not only the differences in the impacts for different fuels, but also includes differences in the size and weights of vehicles that constitute the national vehicle fleet. Greenhouse Gases • Carbon dioxide (CO2) emissions from energy production and use are expressed in tons (short tons) or metric tons (tonnes) of CO2-equivalent (CO2-eq). Although CO2 is the principal greenhouse gas associated with energy use, other gases such as methane, nitrous oxide, black carbon, and SF6, also make some contributions to warming potential. These other con- tributions are converted to an equivalent amount of CO2 with a similar effect and the total is therefore expressed as tonnes of CO2-equivalent. The United States emits about 7 billion tonnes of CO2-equivalent per year, about 6 billion of which is CO2 arising primarily from energy production and use. Average annual CO2 emissions in the United States are about 20 tonnes per person. [Note: Sometimes greenhouse gas emissions are reported in terms of tonnes of “carbon.” One tonne of carbon emissions equals 3.7 tonnes of CO2 emissions, since the weight of CO2 also includes the weight of the oxygen in the molecule.]