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Real Prospects for Energy Efficiency in the United States F Equivalences and Conversion Factors Energy savings are normally measured in megawatt-hours of electricity (MWh) or in million British thermal units (Btu). These energy savings can be converted directly into avoided million tons of emitted carbon (C) or carbon dioxide (CO2). But most people have little feel for these strange units, and so news media, when reporting on energy topics, tend to convert energy and emissions savings to familiar equivalents: namely, avoided cars, homes, or power plants. The tables in the section below show how to perform these conversions, but first it is useful to define the typical car, typical home, and typical power plant—things easily visualized—as used in the conversion tables. A typical car is defined here as one that has an average fuel economy of 24 miles per gallon (mpg) and is driven 12,000 miles per year, for a gasoline use of 500 gallons per year. Such a vehicle would be a passenger car (rather than something larger or heavier such as a van or sport-utility vehicle). A typical home is defined here as one having an average annual electricity use of 12,000 kilowatt-hours (kWh), corresponding to primary energy use at the power plant of 125 million Btu, plus an average annual 75 million Btu of fuel for heat (typically natural gas), for a total of 200 million Btu. Note that, for the discussion below, electricity accounts for about 2/3 of this 200 million Btu.
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Real Prospects for Energy Efficiency in the United States TABLE F.1 Metric Prefixes Unit Multiple Metric Prefix Symbol Value 103 kilo k Thousand 106 mega M Million 109 giga G Billion 1012 tera T Trillion 1015 peta P Quadrillion A typical power plant is defined here as one with a generating capacity of 500 megawatts (MW) that operates for slightly less than 5000 hours per year1 and is thus selling 2.5 billion kWh per year (or 2.5 terawatt-hours [TWh]; tera = 1012 = trillion; see Table F.1). Although a typical 20-year-old power plant has a generating capacity of about 1000 MW, or 1 gigawatt (GW; giga = 109 = billion), the typical power plant as defined here is smaller because newly constructed power plants tend to have a capacity of about 500 MW. The typical uses of energy and electricity given in the definitions above are shown in Column A of Tables F.2–F.4. Table F.1, “Metric Prefixes,” is the basis of all notations used in Tables F.2–F.4. WHICH TABLE TO USE The conversion by energy (Table F.2), electricity (Table F.3), and C or CO2 (Table F.4) differs by up to 50 percent. The choice of which table to use depends on one’s “model.” Those most interested in saving money, primary energy, and air pollution will prefer Table F.2, but those focusing on electricity trade-offs would use Table F.3 (which does not include cars), and those addressing CO2 trade-offs would choose Table F.4. 1 A more accurate number is 4850 hours per year (3300 billion kWh/680 GW from Table 7.1 in Monthly Energy Review, February 2001, and Table 35 in 1999 Electric Power Annual, Volume 2, Energy Information Administration, Department of Energy, Washington, D.C., October 2000, respectively).
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Real Prospects for Energy Efficiency in the United States Table F.2 converts the energy use of cars, homes, and power plants to “primary energy” (also referred to as source energy). Thus, the primary energy associated with electricity production includes the energy burned at the power plant, not just the 30 percent delivered to the home. Of the three conversion tables, Table F.2 would suffice for most purposes. As explained below, Tables F.3 and F.4 give slightly different conversions. Both cost to the customer and air pollution (nitrogen oxide [NOx] and CO2 emissions from combustion, as well as sulfur oxide [SOx] emissions from coal combustion) TABLE F.2 Energy Used Annually by a Typical Car and Home and Generated by a Typical 500 Megawatt Power Plant A Typical Annual Use (rounded) B Conversion to Btua C Annual Energy Use (Btu)b D Energy Use in Units of 1 Million Cars Passenger cars, vans, sport utility vehicles, light trucks—U.S. stock (private and commercial): 247 millionc 1 typical car 500 gald 1 gal = 125,000 Btu 62.5 million — 1 million typical cars 500 million gal 1 gal = 125,000 Btu 62.5 trillion 1 Homes—U.S. stock: 111 millione 1 typical home (electricity + gas/oil) 200 million Btu — 200 million — 1 million typical homes 200 trillion Btu — 200 trillion 3.2 Power plants—U.S. stock: 3,300 TWhf ≡ 1,320 plants (½ GW) Typical power plant (½ GW × 5,000 hours per year) 2.5 TWh 1 kWh = 10,500 Btug 26.2 trillion 0.4 aSee http://bioenergy.ornl.gov/papers/misc/energy_conv.html. bFor metric units (e.g., kWh) the metric prefix is used; for Btu, the “value” multiplier is used, as shown in Table F.1, “Metric Prefixes.” cData from Table 1-11: Number of U.S. Aircraft, Vehicles, Vessels, and Other Conveyances, available at http://www.bts.gov/publications/national_transportation_statistics/html/table_01_11.html. dData from Table MF21 (for motor fuel use) and Table MV1 and MV9 (for private and commercial auto stock) in Highway Statistics 1998, available at http://www.fhwa.dot.gov/ohim/ohimstat.html. Table 1.10 for average annual miles in Monthly Energy Review, DOE/EIA-0035(2000-04), April 2000. eData from Residential Energy Consumption Survey, available at http://www.eia.doe.gov/emeu/. fData from Table 7.5 in Monthly Energy Review, DOE/EIA-0035(2000-04), April 2000. gSee Tables 2.6 and 7.5 in Monthly Energy Review, DOE/EIA-0035(2000-04), April 2000. In 1999, the U.S. electric grid consumed 34.5 quads of source energy to generate and sell 3,300 TWh of electricity. This yields a “heat rate” of 10,500 Btu/kWh.
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Real Prospects for Energy Efficiency in the United States TABLE F.3 Electricity Used Annually by a Typical Home and Generated by a 500 Megawatt Power Plant A Typical Annual Use D Electricity Use in Units of 1 Million Homes 1 typical home 12,000 kWh — 1 million typical homes 12 TWh 1 Typical power plant (½ GW × 5,000 hours per year) 2.5 TWh 0.2 are roughly proportional to primary energy, although the cost per Btu would vary among fuels. Using Table F.2 (Primary Energy) With respect to the use of Table F.2, suppose one learns that low-energy (low-E) windows are saving 1 quad per year (1 quad = 1015 Btu, or 1 quadrillion Btu), which is about 1 percent of total U.S. energy use. One can use Column C of Table F.2 to divide by 1 million cars. Similarly one could calculate 5 million equivalent homes or 38 power plants avoided. Using Table F.3 (Electricity) Suppose one learns, however, that the 2001 refrigerator standard will save 30 billion kWh, or 30 TWh, annually. In this case, Table F.3 indicates that a typical power plant sells 2.5 billion kWh per year (or 2.5 TWh), so it can be seen that as a result of the standard 12 power plants are avoided. Likewise, according to Table F.3, 1 million homes use 12 TWh, and so the standard has freed up electricity to supply 2.5 million homes. But as is noted above, for every 100 Btu of electric energy, homes use another 50 Btu of fuel, so there has not been enough energy and pollution saved to offset 2.5 million homes, but only about 1.7 million.2 2 This 1.7 million home offset can be checked by converting 30 TWh to trillion Btu (using the grid’s heat rate of 10,500 Btu/kWh) and then using Table F.2.
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Real Prospects for Energy Efficiency in the United States Using Table F.4 (CO2) Finally, suppose that manufacturers want to get CO2 credit for the same refrigerator standards, so they use Table F.4 to convert the 12 power plants avoided into 18 million tons of CO2 per year. As before, one can divide 18 million tons of CO2 by the 1 million homes row (11 million tons of CO2) to find about 1.6 million equivalent homes. CO2 savings is often stated in million tons of carbon rather than CO2, so it must be noted that 1 ton of carbon is equivalent to 44/12 = 3.67 tons of CO2. TABLE F.4 CO2 Released by Cars, Homes, and Power Plants A Typical Annual Use (rounded) B Conversion to CO2a C Annual CO2 Release (metric tons) D CO2 Release in Units of 1 Million Cars Passenger cars, vans, sport utility vehicles, light trucks—U.S. stock (private and commercial): 200 millionb 1 typical car 500 galc 1 gal = 8.8 kgd 4.4 1 million typical cars 500 million gal 4.4 million 1 Homes—U.S. stock: 200 millione 1 typical home (electricity + gas/oil) 200 million Btu 1 million Btu = 55 kgf 11 1 million typical homes 200 trillion Btu 11 million 2.5 Power plants—U.S. stock: 3,300 TWhg ≡ 1,320 plants (½ GW) Typical power plant (½ GW × 5,000 hours per year) 2.5 TWh 1 TWh = 0.6 million tonsh 1.5 million 0.34 a1 million tons/quadrillion Btu = 1 kg/million Btu; 1 ton of C corresponds to 3.67 tons of CO2. bData from Tables MV1 and MV9 in Highway Statistics 1998, available at http://www.fhwa.dot.gov/ohim/ohimstat.html. cData from Table MF21 (for motor fuel use) and Tables MV1 and MV9 (for private and commercial auto stock) in Highway Statistics 1998, available at http://www.fhwa.dot.gov/ohim/ohimstat.html. Table 1.10 for average annual miles in Monthly Energy Review, DOE/EIA-0035(2000-04), April 2000. dSee Table B1, p. 104, EIA Emissions of Greenhouse Gases in the US, DOE/EIA-0573(98). eData from Figure 2.1, A Look at Residential Energy Consumption, DOE/EIA-0632(97). fSee Table A19 for million tons of carbon, p. 133, and Table A2 for primary quads, p. 118, Annual Energy Outlook, DOE/EIA-0383(2000). gData from Table 7.5, Monthly Energy Review, DOE/EIA-0035(2000-04). hSee Table A19 for million tons of carbon, p. 133, Annual Energy Outlook, DOE/EIA-0383(2000).
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Real Prospects for Energy Efficiency in the United States A comparison of Column D in Tables F.2 and F.4 shows a slight difference in their equivalence of cars and homes. Table F.2 shows that 1 million homes use as much energy as do 3.2 million cars, but Table F.4 shows that the same 1 million homes produce only as much CO2 as 2.5 million cars. This is because, per Btu, gasoline produces 4/3 as much CO2 as electricity or natural gas. CONVERTING POWER PLANTS (OR PEAK SHAVING) TO “HOMES” In the analysis above, energy (kWh), not power (kW or MW), is discussed. National newspapers often use “1 kW = 1 home” as the relevant conversion factor. This is nearly, but not quite, correct. A more realistic conversion is roughly 1.6 kW for an average California home, and roughly 2.4 kW for an average U.S. home. This is based on the assumption that an average California home uses approximately 8,000 kWh per year, whereas an average U.S. home uses 12,000 kWh. However, owing to fluctuations in power demand, a typical power plant runs for only about 5,000 hours per year rather than 8,760 hours per year.