The Energy Balance of Corn Ethanol: An Update. By Hosein Shapouri, James

A. Duffield, and Michael Wang. U.S. Department of Agriculture, Office of the

Chief Economist, Office of Energy Policy and New Uses. Agricultural Economic

Report No. 814.

Abstract

Studies conducted since the late 1970s have estimated the net energy value (NEV)

of corn ethanol. However, variations in data and assumptions used among the

studies have resulted in a wide range of estimates. This study identifies the factors

causing this wide variation and develops a more consistent estimate. We conclude

that the NEV of corn ethanol has been rising over time due to technological

advances in ethanol conversion and increased efficiency in farm production. We

show that corn ethanol is energy efficient as indicated by an energy output:input

ratio of 1.34.

Keywords: Ethanol, net energy balance, corn production, energy.

About the Authors

Shapouri and Duffield are with USDA’s Office of Energy Policy and New Uses,

300 7th Street, S.W., Room 361 Washington, DC 20024. Wang is with the Center

for Transportation Research, Energy Systems Division, Argonne National

Laboratory, 9700 South Cass Avenue, Argonne, IL 60439. For more information

about this report, call James Duffield, at 202-401-0523; Fax (202) 401-0533; 

E-mail: Jduffield@oce.usda.gov

July 2002
Contents

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iii

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Energy Balance Issue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Estimating Net Energy Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

Data Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

Estimating Energy of Farm Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Estimating Energy for Corn Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Estimating Energy for Ethanol Conversion . . . . . . . . . . . . . . . . . . . . . . . .9

Energy for Ethanol Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Estimating Energy Credits for Coproducts . . . . . . . . . . . . . . . . . . . . . . . .9

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

ii ● The Energy Balance of Corn Ethanol: An Update / AER-814 Office of Energy Policy and New Uses
Office of Energy Policy and New Uses The Energy Balance of Corn Ethanol: An Update / AER-814 ● iii

Summary

Ethanol production in the United States grew from just a few million gallons in the

mid-1970s to over 1.7 billion gallons in 2001, spurred by national energy security

concerns, new Federal gasoline standards, and government incentives. Production

of corn-ethanol is energy efficient, in that it yields 34 percent more energy than it

takes to produce it, including growing the corn, harvesting it, transporting it, and

distilling it into ethanol.

Growth in ethanol production has provided an economic stimulus for U.S. agriculture, because most ethanol is made from corn. The increase in ethanol demand has

created a new market for corn, and agricultural policymakers see expansion of the

ethanol industry as a way of increasing farm income and reducing farm program

payments, while helping the U.S. economy decrease its dependence on imported

oil. Increasing ethanol production induces a higher demand for corn and raises the

average corn price. Higher corn prices can result in reduced farm program

payments.

Today’s higher corn yields, lower energy use per unit of output in the fertilizer

industry, and advances in fuel conversion technologies have greatly enhanced the

energy efficiency of producing ethanol compared with just a decade ago. Studies

using older data may tend to overestimate energy use because the efficiency of

growing corn and converting it to ethanol has been improving significantly over

time. The estimated net energy value (NEV) of corn ethanol was 21,105 Btu/gal

under the following assumptions: fertilizers are produced by modern processing

plants, corn is converted in modern processing facilities, farmers achieve normal

corn yields, and energy credits are allocated to coproducts. 

Moreover, producing ethanol from domestic corn stocks achieves a net gain in a

more desirable form of energy. Ethanol production uses abundant domestic

supplies of coal and natural gas to convert corn into a premium liquid fuel that can

displace petroleum imports.

The initial impetus for ethanol commercialization in the United States came when

the 1970s oil embargoes exposed the vulnerability of U.S. energy supplies. Fuel

ethanol was seen as a gasoline extender; mixing it with gasoline was considered a

means of extending the Nation’s gasoline supply. In the 1980s, ethanol established

a role as an octane enhancer as the Environmental Protection Agency began to

phase out lead in gasoline. Later, ethanol production received a major boost with

the passage of the Clean Air Act Amendments of 1990. Blending ethanol with

gasoline has become a popular method for gasoline producers to meet the oxygen

requirements mandated by the act. Methyl tertiary butyl ether (MTBE), the only

other oxygenate used in the United States, may soon be substantially reduced or

eliminated, because of its propensity to contaminate ground and surface water. The

elimination of MTBE could increase the demand for ethanol significantly.
Office of Energy Policy and New Uses The Energy Balance of Corn Ethanol: An Update / AER-814 ● 1

Introduction

Ethanol production in the United States grew from just

a few million gallons in the mid-1970s to over 1.7

billion gallons in 2001. National energy security

concerns, new Federal gasoline standards, and

Government incentives have been the primary stimuli

for this growth (Lee). In addition, Government and

privately sponsored research has resulted in new technologies that lowered the cost of production of ethanol

made from corn (Hohmann and Rendleman, 1993).

The initial impetus for ethanol commercialization

came during the 1970s. The oil embargo of 1973 and

the Iranian revolution of 1978 caused oil prices to

increase rapidly, creating much concern over the security of national energy supplies. Fuel ethanol became

attractive as a gasoline extender and was considered a

means of increasing the U.S. gasoline supply. About

the same time, the Environmental Protection Agency

(EPA) was looking for a replacement for lead additives

to gasoline used to boost the octane level. Because of

its high octane content, ethanol soon established a role

as an octane enhancer (Lee and Conway). 

In 1990, ethanol production received a major boost

with the passage of the Clean Air Act Amendments

(CAA) of 1990. Provisions of the CAA established the

Oxygenated Fuels Program and the Reformulated

Gasoline (RFG) Program in an attempt to control

carbon monoxide (CO) and ground-level ozone problems. Both programs require certain oxygen levels in

gasoline: 2.7 percent by weight for oxygenated fuel

and 2.0 percent by weight for reformulated gasoline.

Blending ethanol with gasoline has become a popular

method for gasoline producers to meet the new oxygen

requirements mandated by the CAA. Methyl tertiary

butyl ether (MTBE), the only other oxygenate used in

the United States, may soon be substantially reduced

or eliminated, because of its propensity to contaminate

ground and surface water (Blue Ribbon Panel, 1999).

At least 18 States, including California, the State with

the largest consumption of MTBE in the country, are

considering a phase-out of MTBE. There is also

proposed legislation to eliminate it nationwide and

mandate a renewable fuels standard (RFS). An RFS

would increase the demand for ethanol significantly

because corn ethanol is the only viable renewable fuel

sold in the U.S. market today. 

Public policies aimed at encouraging ethanol development are largely motivated by the desire to improve air

quality and enhance energy security. In addition, agricultural policymakers see the expansion of the ethanol

industry as a means of stabilizing farm income and

reducing farm subsidies. Increasing ethanol production

induces a higher demand for corn and raises the

average corn price. Higher corn prices can result in

reduced farm program payments.

Energy Balance Issue

While the Government’s commitment to ethanol has

been welcomed by agricultural interests and the

ethanol industry, critics question the rationale behind

policies that promote ethanol for energy security benefits, stating that corn-ethanol has a negative energy

value (Ho, 1989; Pimentel, 1991; Pimentel and

Pimentel, 1996; Pimentel, 2001). That is, according to

critics, the non-renewable energy required to grow and

convert corn into ethanol is greater than the energy

value present in the ethanol fuel. Thus, they claim that

corn ethanol is not a fossil energy substitute and that

increasing its production does little to displace oil

imports and increase energy security. 

Others argue that although energy balance is of some

concern, it is not the major issue for addressing energy

security. What really matters is that the production of

ethanol can achieve a net gain in a more desirable

form of energy (U.S. Department of Energy, 1980;

Anderson et al., 1988). In other words, abundant

domestic feedstocks such as coal and natural gas can

effectively be used to convert corn into a premium

liquid fuel that replaces imported petroleum. This

approach reduces the energy balance issue to just

looking at the energy value of the liquid fossil fuels

The Energy Balance of

Corn Ethanol: An Update

Hosein Shapouri, James A. Duffield, and Michael Wang
2 ● The Energy Balance of Corn Ethanol: An Update / AER-814 Office of Energy Policy and New Uses

used in the production of corn-ethanol. We use both

approaches in our analysis. 

The energy balance issue first surfaced in the mid1970s when ethanol began to receive attention as a

gasoline extender. Studies during that time that

analyzed the energy benefits of substituting ethanol for

gasoline generally concluded that the net energy value

(NEV, defined as energy content of ethanol minus

fossil energy used to produce ethanol) of corn ethanol

was slightly negative (Ethanol Study Committee,

1979; Chambers et al., 1979). In the late 1980s, the

U.S. desire to reduce air pollution placed ethanol in

the spotlight once again and energy balance studies

resurfaced. About the same time, studies estimating the

emissions of greenhouse gases from ethanol began to

appear in the literature (DeLuchi, 1991; Ho, 1989;

Marland and Turhollow, 1990). Although these studies

focused on estimating the greenhouse gases associated

with ethanol relative to gasoline, some of these studies

also reported the NEV of ethanol. However, there was

a considerable amount of variation in the findings of

these reports. This wide variation relates to various

assumptions about farm production and ethanol

conversion. Furthermore, the various researchers used

data from different time periods. Studies using older

data tended to overestimate energy use because

ethanol manufacturing and farm production technologies have become increasingly energy efficient over

time. To make matters worse, it is often difficult to

determine why results differ from study to study

because the reports often lack certain details on their

calculation procedures. The purpose of this paper is to

identify the methodological differences creating the

inconsistencies among study results and provide a

more consistent estimate for the NEV of corn ethanol.

Table 1 shows the wide variation in the NEV estimates

of several studies. Some studies use lower heating

values (LHV) for measuring energy and others use

higher heating values (HHV). Higher heating value,

also called gross heating value, is the standard heat of

combustion referenced to water in combustion exhaust

as liquid water. Lower heating value, also called net

heat of combustion, is the standard heat of combustion

referenced to water in combustion exhaust as water

vapor. In other words, the difference between HHV and

LHV is the energy associated with condensation of the

water vapor in the combustion products. Although

these two methods can produce slightly different

results, either approach can be used. However, once a

method is chosen, it should be used consistently

throughout the study for all energy calculations.

Table 1—Energy input assumptions of corn-ethanol studies

Study/year Corn yield Nitrogen Nitrogen Corn ethanol Ethanol Total1 Coproducts1 Net1

fertilizer fertilizer conversion rate conversion energy use energy energy 

application production process credits value

rate

Bu/acre lb/acre Btu/lb gal/bu Btu/gal Btu/gal Btu/gal Btu/gal

Pimentel (1991) 110 136 37,551 2.50 73,687 131,017 (LHV) 21,500 -33,517

Pimentel (2001) 127 129 33,547 2.50 75,118 131,062 (LHV) 21,500 -33,562

Keeney and 

DeLuca (1992) 119 135 37,958 2.56 48,470 91,196 (LHV) 8,078 -8,438

Marland and 

Turhollow (1990) 119 127 31,135 2.50 50,105 73,934 (HHV) 8,127 18,154

Lorenz and

Morris (1995) 120 123 27,605 2.55 53,956 81,090 (HHV) 27,579 30,589

Ho (1989) 90 NR NR NR 57,000 90,000 (LHV) 10,500 -4,000

Wang et al. (1999) 125 131 21,092 2.55 40,850 68,450 (LHV) 14,950 22,500

Agri. and Agri-Food

Canada (1999) 116 125 NR 2.69 50,415 68,450 (LHV) 14,055 29,826

Shapouri et al. (1995) 122 125 22,159 2.53 53,277 82,824 (HHV) 15,056 16,193

This study (2002) 125 129 18,392 2.66 51,779 77,228 (HHV) 14,372 21,105

NR: Not reported

LHV: Low heat value = 76,000 Btu per gallon of ethanol. Keeney and DeLuca used 74,680 Btu per gallon of ethanol.

HHV: High heat value = 83,961 Btu per gallon of ethanol. Lorenz and Morris used 84,100 Btu per gallon of ethanol.

1 The midpoint or average is used when studies report a range of values.
Office of Energy Policy and New Uses The Energy Balance of Corn Ethanol: An Update / AER-814 ● 3

Among past studies, Pimentel (1991) reported the

lowest NEV for corn-ethanol. Based on a lower

heating value of energy items, he reported that it

requires a total energy input of about 131,000 British

thermal units (Btu) to produce 1 gallon of ethanol.

Compared with the LHV of 76,000 Btu for ethanol,

this results in a net energy loss of 55,000 Btu per

gallon. Even when coproducts were considered,

Pimentel still estimated a net energy loss of about

33,500 Btu/gal. Pimentel reported identical results in a

1996 publication (Pimentel and Pimentel, 1996).

Pimentel made some minor adjustments in a 2001

publication, but most of the data appeared to come

from the 1991 study and the NEV remained about the

same (table 1). Keeney and DeLuca (1992) also

reported a negative NEV, but their energy deficit was

only about 8,440 Btu/gal. Keeney and DeLuca did not

consider corn-processing byproducts, but they showed

that a positive energy balance could be attained with

low-input corn production. Marland and Turhollow

(1990) reported that it required almost 74,000 Btu

(HHV basis) to produce a gallon of ethanol assuming

that conversion took place in the best ethanol facilities

available then. When energy use is allocated to ethanol

from the coproducts made during the ethanol conversion process, such as gluten meal, gluten feed, and

corn oil, they concluded that the NEV of corn ethanol

was over 18,000 Btu/gal. Results from the Shapouri et

al. (1995) study were very similar to the results of the

Marland and Turhollow study. Lorenz and Morris

(1995) derived the most favorable NEV estimate.

When adding energy coproduct credits, they estimated

a net energy gain of about 30,500 Btu/gal. Agriculture

and Agri-Food Canada (AAFC) (1999) and Wang

(1999) also found relatively high net energy values for

corn ethanol.

Differences among these studies are related to various

assumptions about corn yields, ethanol conversion

technologies, fertilizer manufacturing efficiency, fertilizer application rates, coproduct evaluation, and the

number of energy inputs included in the calculations.

For example, there is about a 64,000 Btu/gal difference

in the results of Pimentel (1991) and Lorenz and

Morris. With respect to growing the corn, Pimentel

reports that it requires 56,720 Btu/gal (LHV)

compared with Lorenz and Morris’s 27,134 Btu/gal

(HHV). Both studies used the same basic inputs, such

as fertilizer, pesticides, and fuel, but Pimentel also

included the energy value embodied in farm

machinery, though he did not present any details on

how he derived embodied energy in farm machinery.

Another factor that makes Pimentel’s estimates higher

is the use of a national average corn yield of only 110

bu/ac, which is characteristic of corn yields seen in

U.S. agriculture in the early 1980s. Lorenz and Morris

used 120 bu/ac, which is based on data from more

recent years. Although Pimentel increased corn yield

in his 2001 report, the NEV remained about the same

as reported in the 1991 study (table 1). 

The time period for which information was collected

on fertilizer plants makes a difference in energy

requirements among the studies. For example, Keeney

and DeLuca reported the highest energy estimate for

fertilizer, almost 38,000 Btu/lb, using source data from

a 1980 study (Dovring and McDowell). More recent

studies, such as Wang et al. (1999), reported about

21,000 Btu/lb, and Lorenz and Morris (1995) reported

about 27,600 Btu/lb. 

Fertilizer application rates can also make a difference

in energy use estimates. For example, Pimentel’s

(1991) nitrogen requirement was 136 pounds per acre,

which is 7 pounds per acre higher than the average

nitrogen rate of the studies reported in table 1.

Pimentel (1991) also reported the highest phosphorus

application rate – 67 pounds per acre compared with

40, 48, 50, and 57 pounds per acre in AAFC (1999),

Lorenz and Morris (1995), Marland and Turhollow

(1990), and Keeney and DeLuca (1992), respectively.

Energy used in ethanol conversion facilities differs

greatly among the studies. For example, the energy

estimate for ethanol conversion in Pimentel’s 1991 and

2001 studies are over 30,000 Btu per gallon higher

than the Wang et al. estimate. Much of this difference

may be related to the data collection periods used.

Estimates used in the Pimentel studies for ethanol

conversion came from data collected in the 1980s

(Energy Research Advisory Board, 1980; and National

Advisory Panel, 1987). Wang et al. used estimates that

reflect today’s ethanol facility, which uses far less

energy than the typical ethanol plant of 10 years ago.

Most ethanol plants in production today have been

extensively modernized and represent near-state-ofthe-art technology. Another major difference between

the Pimentel studies and most other studies is that his

estimates include energy expended on capital equipment. Pimentel’s estimate for converting ethanol is

about 7,000 Btu/gal higher because it includes energy

for steel, cement, and other materials used to construct

the ethanol plant, components not included in most

other studies. Pimentel also used a lower ethanol
4 ● The Energy Balance of Corn Ethanol: An Update / AER-814 Office of Energy Policy and New Uses

conversion rate—2.50 gal/bu. With the exception of

Marland and Turhollow (1990), the other studies use a

conversion rate of 2.53 gal/bu or higher. 

The large variation in coproduct energy credits listed

in table 1 is related to the specific coproducts included

in each analysis. Coproducts depend on the milling

process used for the analysis. Distiller’s dried grains

with solubles (DDGS) is a dry-milling coproduct,

while corn oil, corn gluten meal (CGM), and corn

gluten feed (CGF) are derived from wet milling. Both

dry and wet milling emit carbon dioxide (CO2), but

Lorenz and Morris (1995) are the only authors to estimate a CO2 coproduct credit. Some studies, including

Wang et al. (1999), Lorenz and Morris (1995), and

Shapouri et al. (1995), used both wet-and dry-milling

coproducts and weighted coproduct energy credits

based on the industry average mix of wet- and drymilling plants. Pimentel (1991) and AAFC (1999) used

only DDGS, Keeney and DeLuca (1992) used only a

credit for stillage, and Marland and Turhollow (1990)

used coproducts from wet milling. Ho (1989) gave

energy credits for fusel oil, aldehydes, and DDGS. In

addition to using different coproducts, various

researchers also used different methods for estimating

coproduct values, which have a major influence on the

results. Options for estimating coproduct values are

discussed in more detail below. 

Estimating Net Energy Value 

Estimating the energy input for determining the NEV of

corn-ethanol involves adding up all the nonrenewable

energy required to grow corn and to process it into

ethanol. Most studies, including this one, include only

primary energy inputs in their NEV estimates.

Secondary inputs, such as energy required to build

ethanol facilities, farm vehicles, and transportation

equipment are extremely difficult to quantify. Moreover,

secondary inputs related to the ethanol plant would

account for very little energy on a per gallon basis. This

is because the energy embodied in fixed inputs, such as

the cement used to build the plant, would have to be

distributed over total production (including coproducts)

during the lifetime of the plant. In the case of farm

production, the energy embodied in farm equipment

would have to be distributed over all crops (including

crops not used for ethanol production) for which the

equipment was used over the lifetime of the equipment.

Of the studies listed in table 1, only Pimentel attempted

to quantify the energy embodied in the materials used to

construct an ethanol plant and farm machinery. His

estimates are based on a plant making ethanol from

sugarcane in 1979 (Slesser and Lewis) and farm equipment manufactured in 1976 (Doering). However, few

details are given in Pimentel’s study on the method used

for estimating the energy embodied in construction

materials and farm equipment. 

Data Trends

Reliable data are required to estimate the NEV of corn

ethanol. This analysis uses farm production data from

USDA to estimate energy values for farm inputs such

as gasoline and diesel fuel use, fertilizers, and other

chemicals. It is important that the most current data

available be used to estimate the NEV of ethanol

because the efficiency of growing corn and converting

it to ethanol has improved significantly over the past

20 years. Higher corn yields, lower energy use per unit

of output in the fertilizer industry, and advances in

corn-to-ethanol conversion technologies have greatly

enhanced the economic and technical feasibility of

producing ethanol.

Total energy used in U.S. agriculture, including pesticides, fertilizers, other chemicals, liquid fuels, natural

gas, and electricity, increased from 1,545 trillion Btu

in 1965 to a peak of 2,244 trillion Btu in 1978, and

then steadily declined to a low of 1,548 trillion Btu in

1989 (USDA, 1997). Since 1989, there has been a

slight upward movement in energy use, but still well

below the peak levels that occurred in the 1970s.

Agricultural energy use was about 1,800 trillion Btu in

1999, but 2000 energy use dropped to about 1,600 trillion Btu when higher energy prices caused farmers to

conserve. The decline in agricultural energy use since

1978 is largely attributed to the replacement of gasoline-powered farm machinery with more fuel-efficient

diesel engines (Uri and Day, 1991). 

While energy use has been declining, there has been a

rising trend in corn yields. Figure 1 shows that with

the exception of a few bad years, annual corn yields

have been increasing since 1975. The large downward

spikes in 1983, 1988, and 1993 were caused by

adverse weather. Droughts caused unusually low yields

in 1983 and 1988, and in 1993 the Midwest experienced a devastating flood. Higher yields without corresponding increases in energy use indicate that farm

resources are being used more efficiently. 

Fertilizer use in grain production rose for many years,

peaking in the early 1980s when it began to decline.

Nitrogen use per planted acre of corn declined from
Office of Energy Policy and New Uses The Energy Balance of Corn Ethanol: An Update / AER-814 ● 5

140 pounds in 1985 to 132 pounds in 2000 (Taylor,

1994; USDA/NASS). Phosphate use declined from 60

to 47 pounds per acre, and potash use declined from

84 pounds to 51 pounds per acre during the same

period. In addition, the manufacture of agricultural

chemicals has become more energy efficient. The

fertilizer industry, for example, has undergone a major

technological advancement in the last 20 years, and

U.S. farmers have gained substantial real energysaving benefits in terms of nitrogen and phosphorus

(Bhat et al., 1994). Energy savings in nitrogen production has been especially important since it has a much

higher average energy requirement than phosphorous

and potash fertilizers. Bhat et al. (1994) reported that

the energy consumed in producing nitrogen fertilizers

declined about 11 percent from 1979 to 1987. 

Making ethanol from corn also has become more

energy efficient. Hohmann and Rendleman (1993)

reported that a shift in production to larger plants and

the adoption of energy-saving innovations reduced the

processing energy required to produce a gallon of

ethanol from 120,000 Btu in 1981 to 43,000 Btu in

1991. Efforts by the industry to conserve electricity

have also resulted in substantial energy savings.

Modern plants are conserving energy by utilizing

cogeneration units that produce steam and electricity

simultaneously. Advances in alcohol dehydration have

also resulted in considerable energy savings (Hohmann

and Rendleman, 1993). 

Estimating Energy of Farm Inputs

Estimates of farm energy use in this study are based

on data from the 1996 Agricultural Resource

Management Survey (ARMS), the most recent data

available from USDA. ARMS (formerly called the

Farm Costs and Returns Survey (FCRS)), is conducted

every 5 years and provides data from selected States

on fuel, electricity, natural gas, fertilizer, and chemicals used on the farm and activities of moving farm

products to initial storage facilities (Ali and McBride,

1994). We focused our analysis on the major cornproducing States: Illinois, Indiana, Iowa, Minnesota,

Nebraska, Ohio, Michigan, South Dakota, and

Wisconsin. These nine States account for about 80 and

91 percent of U.S. corn and ethanol production,

respectively. We weighted farm input use by corn

acreage planted in each State to estimate an average

input level for corn production. 

There is a considerable amount of variation in State

energy use estimates between survey years. This variation illustrates the unpredictable effect that weather

has on energy use and other aspects of production

agriculture. For example, the 1991 FCRS data used in

the Shapouri et al. (1995) study were considerably

different from the data collected by the 1996 ARMS

(tables 2 and 3). The 1991 corn crop was hampered by

dry weather during the summer months, resulting in

lower yields. Production totaled 7.5 billion bushels,

about 6 percent below the 1990 crop. The U.S. average

yield was about 109 bushels per harvested acre, down

Table 2—Energy-related inputs used to grow corn in nine States and nine-State weighted average, 1991

Nine-State

weighted

IL IN IA MN NE OH MI SD WI average

Yield (1990-92) Bushels/acre 128 120 130 119 130 120 110 79 114 121.9

Seed Kernels/acre 25,384 24,827 25,150 26,804 26,546 26,185 25,274 22,115 26,310 25,502

Fertilizer:

Nitrogen Pounds/acre 156 143 119 79 142 122 127 68 107 124.5

Potash Pounds/acre 78 64 47 55 23 59 47 26 63 52.77

Phosphate Pounds/acre 90 108 49 57 3 91 63 11 45 58.17

Lime Pounds/acre 480 340 280 40 0 140 680 0 120 242.18

Energy:

Diesel Gallons/acre 4 5 4 5 18 5 7 6 8 6.85

Gasoline Gallons/acre 4 4 3 3 4 3 3 3 3 3.4

LPG Gallons/acre 2 2 5 4 4 4 3 5 2 3.42

Electricity kWh/acre 12 28 5 28 97 10 11 86 69 33.59

Natural gas Cubic ft/acre 60 10 0 0 1,610 10 50 0 10 245.97

Custom work Dol./acre 8 6 7 5 6 5 4 4 16 6.68

Chemicals Dol./acre 23 28 24 21 23 21 21 14 21 23

Custom drying Dol./acre 1 2 2 2 2 1 2 0 1 1.79

Source: USDA, Economic Research Service and Office of Energy Policy and New Uses
6 ● The Energy Balance of Corn Ethanol: An Update / AER-814 Office of Energy Policy and New Uses

10 bushels from 1990 and 30 bushels lower than the

record yield of 1994. Although yield was down in

1991 due to poor summer weather, the corn crop was

aided by good maturing and harvesting weather during

the fall. Dry ground conditions in the fall allowed

farmers to get their equipment in the fields at the ideal

time, and 93 percent of the corn crop was harvested by

November 1. In addition to good harvest conditions,

the dry weather reduced the moisture content of the

1991 corn crop. This helped the NEV of ethanol,

because a relatively small amount of energy was

needed for drying corn in 1991. 

When the survey was conducted again in 1996, the

U.S. corn yield was about 127 bushels per acre, up

13.6 bushels from 1995, but 11.5 bushels below the

record of 1994. A cool wet spring across the Corn Belt

in 1996 slowed corn growth and many farmers delayed

planting until the end of spring. To lengthen the

growing season, the corn harvest was slowed in the

Midwest to give the crop more time to mature and dry.

Although the season ended later than usual in most

States, the 1996 corn crop had a high moisture content,

and the energy needed for drying was abnormally

high. This is reflected in the increase in electricity,

liquefied petroleum gas (LPG), natural gas, and diesel

fuel use in some States from 1991 to 1996 (tables 2

and 3). Thus, using the 1996 farm survey data may

overestimate the average energy used on the farm in

recent years. Ideally, we would prefer to use annual

average energy use during the last several years, but

such data are not available. 

Yield is a critical part of the NEV calculation—for

every 1 percent increase in yield, the NEV of ethanol

increases 0.37 percent, ceteris paribus. Although yield

has been generally rising over time, the annual variation

is very volatile (fig. 1). Therefore, we used a 3-year

average yield instead of the average yield for the survey

year. The 1990-92 average corn yield, weighted by corn

production in each State, was used to convert farm

inputs from a per acre basis to a per bushel basis (table

2). The farm energy use estimates for 1996 are based on

the nine-State 1995-97 average corn yield (table 3).

Table 3—Energy-related inputs used to grow corn in nine States and nine-State weighted average, 1996

Nine-State

weighted

IL IN IA MN NE OH MI SD WI average

Yield (1995-97) Bushels/acre 126 120 133 126 129 123 115 94 119 125

Seed Kernels/acre 25,384 24,827 25,150 26,804 26,546 26,185 25,274 22,115 26,310 25,495

Fertilizer:

Nitrogen Pounds/acre 160 134 128 117 137 148 118 78 80 129.38

Potash Pounds/acre 102 87 58 46 5 89 99 7 48 59.25

Phosphate Pounds/acre 71 54 44 50 25 74 48 27 33 48.16

Lime Pounds/acre 20 20 20 0 0 20 20 0 60 15.35

Energy:

Diesel Gallons/acre 7 5 7 8 18 6 8 5 9 8.6

Gasoline Gallons/acre 3 3 3 3 4 3 3 2 2 3.09

LPG Gallons/acre 5 8 7 10 4 15 5 3 3 6.36

Electricity kWh/acre 15 101 154 91 82 41 51 16 20 77.13

Natural gas Cubic ft/acre 150 550 50 0 560 0 250 10 270 200

Custom work Dol./acre 14 14 14 15 17 14 15 17 15 15.07

Chemicals Dol./acre 29 29 29 26 19 29 26 19 26 26.00

Source: USDA, Economic Research Service and Office of Energy Policy and New Uses.

Figure 1

U.S. corn yield

Bushels/acre

Source: National Agricultural Statistics Service, USDA.

1969 74 79 84 89 94 99

60

70

80

90

100

110

120

130

140

150
Office of Energy Policy and New Uses The Energy Balance of Corn Ethanol: An Update / AER-814 ● 7

Nebraska had the highest energy use in both survey

years, primarily due to the energy used for irrigation.

More than 78 percent of the corn acres harvested in

Nebraska required irrigation in 1996. In contrast, only

5 percent of the corn acreage was irrigated in Iowa and

no corn acreage was irrigated in Indiana, Illinois, and

Wisconsin. The impact of irrigation requirements on

energy use in Nebraska was especially apparent during

1991’s extremely dry summer. Over 1,600 cubic feet of

natural gas, used to power irrigation pumps, were

required per acre of corn in Nebraska in 1991 (table 2).

This was almost three times more natural gas than was

required in 1996 (table 3). Electricity, another energy

source for irrigation pumps in Nebraska, was also

unusually high in 1991. The relatively high electricity

use estimate in South Dakota in 1991 was also probably the result of increased irrigation needs–almost all

irrigation pumps in South Dakota are powered by electricity. In 1996, when less irrigation water was

required, the amount of electricity used to grow a

bushel of corn in South Dakota decreased significantly. 

Other major factors that caused variation in the State

energy use estimates were drying requirements and

fertilizer use. There was a significant rise in total

energy use in 1996 in Indiana, Iowa, Minnesota, and

Ohio, largely because of the increased use of LPG, electricity, and natural gas required for drying crops.

Minnesota and Ohio corn acreage also required significantly more energy in the form of nitrogen fertilizer in

1996. On the other hand, Wisconsin reduced energy

consumption in 1996 because of significantly less fertilizer use and lower electricity requirements for irrigation.

Table 4 reports farm energy-related inputs on a Btu per

bushel of corn basis for each State and a nine-State

weighted average for 1996. The inputs are first

converted to Btu/bu of energy by multiplying each

input by its high heat energy value, e.g., a gallon of

diesel fuel has 137,200 Btu, a gallon of gasoline has

125,070 Btu, and a cubic foot of natural gas has 1,020

Btu. One kilowatthour of electricity has 8,625 Btu,

after adjusting for the U.S. average Btu loss during

electricity generation. Estimates for electricity generation are based on a weighted average of all sources of

power, including coal, natural gas, nuclear, and hydroelectric. We then determined how much energy is

required from each input to produce a bushel of corn.

All thermal inputs and outputs in this study are measured on a higher heating value basis. The energy

required for hauling these inputs to the farm from local

retailers was also estimated (table 4). Estimates for

transporting the corn from farms to the first storage

facilities are included in the diesel fuel estimate in

table 4. Electricity used on the farm is adjusted for

transmission loss by a factor of 1.087, according to

data on electricity losses during transmission and

distribution from the Energy Information

Administration (Wang et al., 1999).

The amount of fertilizer applied to corn is provided by

the ARMS and converted to Btu. The actual amount of

Table 4—Total energy requirements of farm inputs for nine States and nine-State weighted average, 1996

Nine-State

weighted

IL IN IA MN NE OH MI SD WI average

Btu to produce one bushel of corn

Seed 227 245 220 259 225 286 290 268 287 242

Fertilizer use:

Nitrogen 23,372 20,579 17,660 17,118 19,491 22,091 18,872 15,297 12,303 19,082

Phosphate 1,154 911 679 805 401 1,222 852 578 559 789

Potassium 3,024 2,700 1,642 1,367 133 2,720 3,236 275 1,525 1,776

Lime 117 123 111 0 0 120 128 0 371 90

Energy:

Diesel 8,887 6,171 8,884 10,953 22,697 7,502 11,520 8,657 12,856 11,175

Gasoline 3,181 4,143 4,065 3,958 4,251 4,168 4,566 3,752 2,925 3,859

LP gas 4,285 6,744 5,233 7,922 2,840 12,247 4,713 3,160 2,213 5,200

Natural gas 1,292 4,973 408 0 4,710 0 2,359 115 2,462 1,768

Electricity 1,125 7,895 10,849 6,798 5,929 3,146 4,117 1,642 1,540 5,665

Chemicals 4,251 4,899 3,633 3,160 3,381 4,823 4,022 3,237 3,053 3,797

Custom work 3,146 3,303 2,981 3,236 3,634 3,223 3,545 4,987 3,426 3,366

Input hauling 920 808 619 556 427 884 815 393 609 663

Total energy 54,980 63,494 56,984 56,133 68,120 62,433 59,034 42,360 44,128 57,476

Source: USDA, Economic Research Service and Office of Energy Policy and New Uses.
8 ● The Energy Balance of Corn Ethanol: An Update / AER-814 Office of Energy Policy and New Uses

pesticides (herbicides and insecticides) applied to corn

acreage is not provided by the ARMS, but the survey

does collect information on pesticide expenditures.

These expenditures were converted to pounds of pesticides based on the number of acres treated and prices

in that year. Pounds of pesticides were then converted

to Btu. Table 5 shows energy used to produce agricultural fertilizers and pesticides. The data for energy use

of manufacturing fertilizers and pesticides are from

Argonne’s Green House Gases Regulated Emissions

and Energy Use in Transportation (GREET) model.

Argonne has developed this full fuel-cycle model to

estimate energy use and emissions from transportation

fuel/vehicle technology systems. The model includes

detailed information on corn farming and chemical

manufacturing. The model and its documents are

posted at http://greet.anl.gov. Energy estimates for

manufacturing nitrogen fertilizer and phosphoric acid

were derived from the Fertilizer Institute Production

Cost Surveys (2000). Energy used for producing lime

is 620 Btu/lb (Blankenhorn et al., 1985).

Energy use for transporting fertilizers and pesticides was

calculated with the following assumptions. We assumed

that the transportation energy is diesel fuel (table 4). A

transportation distance of 400 miles for barge and 750

miles for rail was assumed for transporting chemicals

from manufacturing plants to bulk terminals. A transportation distance of 50 miles was assumed for Class 8

trucks to transport from bulk terminals to mixing centers.

A transportation distance of 30 miles was assumed for

Class 6 trucks to transport from mixing centers to corn

farms. With these assumptions, we applied Argonne’s

GREET model to calculate a transportation energy use

of 301 Btu/lb for chemical transportation. 

The energy value of growing seed is assumed to be

equal to 150 percent of the energy required to grow

corn. The nine-State weighted average kernels per

acre is 25,495 (table 3). Corn seed uses more energy

than regular corn because there is an additional

storage and packaging cost. Also, it takes more energy

to haul the seed from a local seed farm to retailers and

from retailers to corn farmers. Energy used for

planting the seed and other farm activities such as

land preparation, plowing, weeding, distribution of

fertilizer and chemicals, irrigating, harvesting, and

drying, is included in the total farm fuels and electricity estimates (table 4). 

The estimates in table 4 also include the energy used

to mine, extract, and manufacture the raw materials

into the final energy product. The sum of these

energy values was included in the estimates to derive

the total energy associated with each farm input

required to produce a bushel of corn. Input efficiencies for fossil energy sources, which were estimated

with Argonne’s GREET model, were used to calculate these additional energy input values. In particular, GREET estimated the energy efficiency of

gasoline (80.5 percent), diesel fuel (84.3 percent),

LPG (89.8 percent), natural gas (94.0 percent), coal

(98.0 percent), and electricity (39.6 percent). After

adjusting the inputs by these energy efficiencies, the

total energy required to produce a bushel of corn in

1996 was 57,476 Btu (table 4). 

Estimating Energy for Corn Transport

We made the following assumptions to calculate

energy use for transporting corn from farms to 

ethanol plants:

• A distance of 40 miles for class 8 trucks from

collectors to terminals,

• A distance of 350 miles for barges from terminals

to ethanol plants,

• A distance of 400 miles for rail from terminals to

ethanol plants. 

As mentioned earlier, the farming survey data already

included energy use for transporting corn from farms

to initial storage facilities, typically a local grain

elevator. The energy associated with transporting the

corn from local storage facilities to ethanol plants was

estimated by the GREET model. The average energy

Table 5—Energy used to produce fertilizers, herbicides, and insecticides

Nitrogen1 Phosphoric acid1 Potash2 Herbicides2 Insecticides2

Diesel (Btu/lb) 0 0 642 67,310 69,299

Natural gas (Btu/lb) 16,857 56 559 25,802 26,564

Electricity (kWh/lb) 0.094 0.215 0.255 5.589 5.755

1 Fertilizer Institute, 2000. 

2 GREET model, Agonnne National Laboratory, 2001
Office of Energy Policy and New Uses The Energy Balance of Corn Ethanol: An Update / AER-814 ● 9

used for transporting a bushel of corn was 6,020 Btu

or about 2,263 Btu per gallon of ethanol (table 6). 

Estimating Energy for Ethanol Conversion 

Ethanol production facilities include both wet-milling

and dry-milling operations. Dry mills are usually

smaller and are built primarily to manufacture ethanol.

Wet mills are “corn refineries,” producing a host of

high-value products such as high-fructose corn syrup

(HFCS), dextrose, and glucose syrup. Since both wet

and dry milling are used to convert corn to ethanol,

our energy conversion estimates are weighted accordingly. Wet milling accounts for about 55 percent of

U.S. ethanol production, and dry milling accounts for

about 45 percent. 

Thermal and electrical power are the main types of

energy used in both types of milling plants. Currently,

most wet-milling plants generate both electrical and

thermal energy from burning natural gas and coal.

Most dry-milling plants generate only steam, requiring

that they purchase electricity from a utility. Electricity

is used mostly for grinding and running electric

motors. Thermal energy is used for fermentation,

ethanol recovery, and dehydration. Flue gas is used for

drying and stillage processing. 

Estimates of the energy used to convert corn to ethanol

is based on a U.S. industry survey conducted in

September 2001 by BBI International. The survey was

commissioned by the Office of Energy Policy and

New Uses, USDA. BBI International collected information from ethanol plants on thermal and electrical

energy used per gallon of ethanol for both dry and wet

ethanol plants. The survey was conducted by telephone

interviews with 17 dry-mill ethanol plants. The

number of wet-mill ethanol plants cannot be disclosed

in order to protect the confidentiality of the wet-mill

producers in the survey. The total production capacity

of the plants in the survey is over 1.3 billion gallons or

about 65 percent of industry’s current capacity.

On average, dry-mill ethanol plants used 1.09 kWh of

electricity and over 36,000 Btu of thermal energy per

gallon of ethanol. With energy losses to produce electricity and natural gas considered, the average dry-mill

ethanol plant consumed about 48,772 Btu of primary

energy per gallon of ethanol produced (table 6). Wetmill ethanol plants participating in the survey used an

average 51,060 Btu per gallon of coal and natural gas

(with more plants using natural gas) to produce the

steam and electricity used in the plants. After adjustments for energy losses to produce coal, natural gas,

and electricity, on average, a wet-mill ethanol plant

used 54,239 Btu of primary energy to make a gallon of

ethanol (table 6). The average conversion rate for dry

mills is 2.64 gallons per bushel and 2.68 gallons per

bushel for wet mills.

Energy for Ethanol Distribution 

We made the following assumptions for calculating

energy use to transport ethanol from ethanol plants to

refueling stations. A distance of 80 miles was assumed

for trucks to transport ethanol from ethanol plants to

collection terminals; a distance of 520 miles for barge

from collection terminals to fuel distribution terminals;

a distance of 800 miles for rail from collection terminals to distribution terminals; and a distance of 25

miles for trucks from distribution terminals to refueling stations. With the GREET model, we estimated

an energy use of 1,588 Btu per gallon of ethanol transported for both wet and dry milling (table 6).

Estimating Energy Credits for Coproducts 

The coproducts used in this analysis include distiller’s

dried grains (DDGS) with solubles from dry milling,

and corn oil, corn gluten meal (CGM), and corn gluten

feed (CGF) from wet milling. There are basically four

ways to estimate energy credits for coproducts. First,

the energy content of coproducts can be used to estimate energy credits. For example, a pound of corn

gluten meal or corn gluten feed has a caloric content of

8,000 Btu. This results in about a 40-percent coproduct

energy credit. The disadvantage of this method is that

calories are a measurement of food nutritional value

and are not a good proxy for energy in a fuel context.

A second method of estimating coproduct energy

values is to use the relative market values of ethanol

Table 6—Energy use and net energy value per gallon

without coproduct energy credits, 1996

Milling process

Production phase Dry Wet Weighted average

Btu per gal

Corn production 21,803 21,430 21,598

Corn transport 2,284 2,246 2,263

Ethanol conversion 48,772 54,239 51,779

Ethanol distribution 1,588 1,588 1,588

Total energy used 74,447 79,503 77,228

Net energy value 9,513 4,457 6,732

Energy ratio 1.11 1.04 1.08
and its coproducts. For example, if energy used to

produce ethanol is allocated between ethanol and

coproducts based on their 10-year average market

values, about 30 percent of energy used to produce

ethanol should be assigned to the coproducts. The

problem with this method is that prices of ethanol and

ethanol coproducts are determined by a large number

of market factors that are unrelated to energy content.

Third, one can allocate energy use among multiple

products on an output weight basis, regardless of the

operation’s purpose or the coproducts’ economic

values. If energy used to produce ethanol is allocated

between ethanol and coproducts based on the output

weight, about 48 percent of energy used to produce

ethanol could be assigned to the ethanol and 52

percent to coproducts. The problem with this method

is that the weight of a product is not always a good

measurement of its energy value. 

A fourth method, based on the replacement value of

coproducts, is the method chosen for our final results.

Energy credits are assumed to be equal to the energy

required to produce a substitute for the ethanol

coproduct. In this analysis, we used soybean meal as

the substitute for distiller’s grain with solubles, corn

gluten meal and corn gluten feed, and soybean oil was

used as the substitute for corn oil. Data from the 1991

FCRS soybean survey was used to help estimate the

energy value of the soybean coproducts (Ali and

McBride, 1994). Using this method, about 19 percent

of the energy used to produce ethanol would be

assigned to coproducts. This method has appeal

because the coproduct value is measured by energy

units unlike the other methods that use non-energy

units. Also, since energy replacement values result in

less energy credits than the other methods, it can be

considered a conservative estimate.

Results 

Table 6 summarizes input energy requirements by phase

of ethanol production on a Btu per gallon basis for 1996

without coproduct credits. It includes energy losses

from line loss, venting losses at the ethanol plant, and

energy associated with mining, refining, and transporting raw materials used in energy production. Energy

estimates are provided for both wet and dry milling, as

well as a weighted average of these two ethanol production processes. In each case, corn ethanol has a positive

energy balance, even before adding coproduct energy

credits. Table 7 presents the final NEV results with

coproduct energy credits. For comparative purposes, the

coproduct energy values are shown for each of the four

methods described above. However, from this point

forward, we will limit our discussion to the replacement

value case, which is our preferred method for measuring

coproduct energy value. The NEV estimate for corn

ethanol produced from wet milling is 19,262 Btu per

gallon, the NEV estimate for dry milling is 22,629 Btu

per gallon, and the weighted average is 21,105 Btu per

gallon (table 7). The energy ratio is 1.30 and 1.37 for

Table 7—Net energy value per gallon of ethanol and energy ratio with coproduct energy credits, 1996

Energy allocation Energy use

Energy Coproduct with coproduct NEV with Energy

Ethanol Coproduct use credit credit coproducts ratio

Percent -------------------------------- Btu/gal---------------------------------

Output weight basis:

Wet mill 48 52 79,503 40,516 38,987 44,974 2.15

Dry mill 49 51 74,447 37,158 37,289 46,672 2.25

Weighted average 48 52 77,228 39,333 37,895 46,066 2.22

Energy content:

Wet mill 57 43 79,503 33,503 46,000 37,961 1.83

Dry mill 61 39 74,447 28,415 46,032 37,929 1.82

Weighted average 58 42 77,228 31,769 45,459 38,502 1.85

Market value:

Wet mill 70 30 79,503 23,374 56,129 27,832 1.50

Dry mill 76 24 74,447 17,486 56,961 27,000 1.47

Weighted average 72 28 77,228 21,179 56,049 27,912 1.50

Replacement value:

Wet mill 81 19 79,503 14,804 64,699 19,262 1.30

Dry mill 82 18 74,447 13,115 61,332 22,629 1.37

Weighted average 81 19 77,228 14,372 62,856 21,105 1.34

10 ● The Energy Balance of Corn Ethanol: An Update / AER-814 Office of Energy Policy and New Uses
Office of Energy Policy and New Uses The Energy Balance of Corn Ethanol: An Update / AER-814 ● 11

wet and dry milling, respectively, and the weighted

average energy ratio is 1.34.

As discussed earlier, some researchers prefer

addressing the energy security issue by looking at the

net energy gain of ethanol from a liquid fuels standpoint. In this case, only the liquid fossil fuels used to

grow corn and produce ethanol are considered in the

analysis. On a weighted average basis, about 83

percent of the total energy requirements come from

non-liquid fuels, such as coal and natural gas. The

liquid fuels, which include gasoline, diesel, and fuel

oil, account for about 21,700 Btu per bushel.

Calculations based on liquid fuel use provide an estimate of the petroleum displacement value of ethanol.

Comparing the energy input value of liquid fuels to the

total Btu output value of ethanol indicates a net energy

gain of about 70,600 Btu for every bushel of corn used

in the production of ethanol. In other words, one Btu

of liquid fossil fuel, used in combination with other

forms of energy, can produce 6.34 Btu of fuel ethanol. 

Discussion 

When comparing this study with the other studies in

table 1, the results are similar to those of Marland and

Turhollow (1990) and Wang et al. (1999). The NEV

estimates from these three studies range from about

18,000 Btu/gal to almost 23,000 Btu/gal. The NEV

estimated in this study is almost 5,000 Btu per gal

greater than the NEV estimated by Shapouri et al.

(1995), which used 1991 ARMS data to estimate the

energy used in farm production. The higher NEV in

this study is partly due to a higher average corn yield

that lowered the energy input used per acre. Moreover,

increased energy efficiency in fertilizer production and

other agricultural chemicals, and the adoption of

energy-saving technologies in corn ethanol conversion,

increased the NEV in the latter study (table 1). 

Lorenz and Morris’s NEV estimate is over 9,000

Btu/gal greater than our NEV, but much of this difference can be explained by the large value they use for

coproduct energy credits. They are the only authors to

use carbon dioxide as an energy coproduct, which

adds 4,460 Btu/gal to their NEV. Only a few ethanol

facilities are selling carbon dioxide today, so we did

not include it in our analysis. The Keeney and DeLuca

study reported a negative NEV, but they reported a

very low value for energy coproducts. They used only

a stillage credit and did not include processing coproducts, such as CGF, CGM, and corn oil. Adding these

coproduct credits to their NEV estimate would raise

their NEV estimate significantly. They also appear to

have used an outdated estimate for the energy used for

manufacturing nitrogen fertilizer. Adjusting their

nitrogen fertilizer estimate to reflect modern technology and adding processing coproducts to their

calculations would likely result in a positive NEV. Ho

(1989) also reported a negative NEV, but his energy

deficit is only 4,000 Btu/gal. Ho used an unusually

low corn yield of 90 bushels per acre. Looking at

figure 1, it is apparent that this yield would represent

only very poor years, like 1988 when U.S. agriculture

experienced a serious drought. If Ho had used a yield

that reflected a normal year, his NEV estimate would

have been significantly higher. 

Pimentel reported the lowest NEVs by far, about

-33,500 Btu/gal. There is a difference of more than

50,000 Btu between Pimentel’s NEV and the estimate

derived in this study (table 1). Many factors

contributed to Pimentel’s low estimate. For example,

with the exception of Ho, Pimentel’s 1991 study used

the lowest corn yield among the studies. His 1991

study used the highest fertilizer application rate and

the lowest corn ethanol conversion rate. He increased

corn yield and reduced fertilizer application rate in his

2001 study, but oddly, the NEV in the latter study

went down. His estimate for energy used for nitrogen

fertilizer processing was extremely high and appears

not to reflect technology used by modern facilities.

The amount of energy required for ethanol conversion

in Pimentel’s studies also appears outdated.

Conversion estimates used by the other studies ranged

between 40,850 Btu/gal (LHV) and 57,000 Btu/gal

(LHV), while Pimentel’s studies calculated about

75,000 Btu (LHV) to convert a gallon of ethanol. In

addition, he is the only author to include an energy

value for steel, cement, and other materials used in the

production of equipment, farm vehicles, and the

ethanol plant.
12 ● The Energy Balance of Corn Ethanol: An Update / AER-814 Office of Energy Policy and New Uses

Conclusions 

We conclude that the NEV of corn-ethanol is positive

when fertilizers are produced by modern processing

plants, corn is converted in modern ethanol facilities,

and farmers achieve average corn yields. Our NEV

estimate of over 21,000 Btu per gallon could be

considered conservative, since it was derived using the

replacement method for valuing coproducts, and it

does not include energy credits for plants that sell

carbon dioxide. Corn ethanol is energy efficient, as

indicated by an energy ratio of 1.34; that is, for every

Btu dedicated to producing ethanol there is a 34-

percent energy gain. Furthermore, producing ethanol

from domestic corn stocks achieves a net gain in a

more desirable form of energy, which helps the United

States to reduce its dependence on imported oil.

Ethanol production utilizes abundant domestic energy

feedstocks, such as coal and natural gas, to convert

corn into a premium liquid fuel. Only about 17 percent

of the energy used to produce ethanol comes from

liquid fuels, such as gasoline and diesel fuel. For every

1 Btu of liquid fuel used to produce ethanol, there is a

6.34 Btu gain. 

When looking at past NEV studies, it appears that

energy requirements for producing a gallon of ethanol

are falling over time. One of the primary factors for

this increase in energy efficiency is the increase in

U.S. corn yields. When ethanol first emerged as a

gasoline extender in the 1970s, corn yield was averaging about 90 bushels per acre. This study used

1995-97 average corn yield of 125 bushels per acre,

which is about 39 percent greater than the yields of the

1970s. Corn yields continue to rise in the United

States—the average corn yield per acre for the past 3

years (1999-2001) was about 135 bushels per acre. If

the 1999-2001 average corn yield were used in this

analysis, the total energy used to produce a bushel of

corn would decline by more than 4,200 Btu. As corn

yields increase over time, we can expect the energy

balance of corn ethanol to increase, as well. Other

major factors causing this increase in energy efficiency

are related to the energy-saving technologies adopted

by ethanol producers and manufacturers of fertilizers

and other farm inputs. Higher energy costs will likely

continue to provide incentives for these industries to

become more energy efficient, which will continue to

push the NEV of corn ethanol higher.
Office of Energy Policy and New Uses The Energy Balance of Corn Ethanol: An Update / AER-814 ● 13

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Glossary

AAFC Agriculture and Agri-Food Canada

ARMS Agricultural Resources Management Survey

Btu British thermal units

CAA Clean Air Act Amendments

CO Carbon monoxide

CO2 Carbon dioxide

CGF Corn gluten feed (21 percent protein)

CGM Corn gluten meal (60 percent protein)

DDGS Distiller’s dried grains with solubles

EPA U.S. Environmental Protection Agency

FCRS Farm Costs and Returns Survey

GHG Greenhouse gases

GREET Greenhouse gases, regulated emissions, and energy use in transportation

HHV High-heat value

HFCS High-fructose corn syrup

KWh Kilowatthour

LHV Low-heat value

LPG Liquefied petroleum gas

MTBE Methyl tertiary butyl ether

NASS National Agricultural Statistics Service

NEV Net energy value

RFG Reformulated gasoline

USDA U.S. Department of Agriculture