The National Academies: Advisers to the Nation on Science, Engineering, and Medicine

Supply and Demand

Two profound questions loom over all other energy concerns: Will we have enough affordable energy in the near future? What will we do for the long term?

The answers depend on our inventory of sources. At present, oil accounts for 40% of total energy consumption in the United States. Coal provides 23% and natural gas provides 22% of our energy. Another 8% comes from nuclear power plants. Renewable energy sources round out the roster, accounting for 7% of consumption – mostly as the result of hydropower investments made in the last century and the use of biomass (organic matter such as wood, municipal waste, and agricultural crops) for energy production.

Those sources and their proportions will have to change eventually, since the planet's known supplies of fossil fuels are limited. But during the next couple of decades, the nation's energy menu is unlikely to be substantially different from today's – assuming “business as usual” conditions.

That may be a lot to assume: Energy prices and availability aren't solely determined by the size of the supply. They're also affected by the economy, possible new laws and regulations governing energy choices (such as emissions of carbon dioxide and other gases), worldwide demand, the policies and political stability of petroleum-rich nations, lifestyle choices and business decisions, climate change, and the pace of developments in science and engineering. Any of these factors can change in a very short period of time.

Relative contributions of energy sources to total U.S. energy consumption in 2006.

Relative contributions of energy sources to total U.S. energy
consumption in 2006.13

Still, if the economy and the inflation rate perform as expected and there are no drastic geopolitical changes or dramatic technological breakthroughs, objective forecasts show that traditional supplies of petroleum, gas, and coal will be adequate to meet expanded demand for decades.


The United States, with less than 5% of the world's population, is home to one-third of the world's automobiles. Over the next 20 years, the total number of miles driven by Americans is forecasted to grow by 40%, increasing the demand for fuel.14 Yet there is little that can be done locally to increase the oil supply. U.S. domestic production of crude oil peaked around 1970 at about 9.5 million barrels per day (MBD) and had declined to 5.1 MBD by 2006.15 Today America imports almost two-thirds of its oil from a handful of nations.16 The Energy Information Administration (EIA), a U.S. government agency that provides official energy statistics and forecasts, expects U.S. production of oil to remain approximately constant through 2030, while imports are projected to rise gradually to about 70% of consumption.17

Electricity, the #1 Secondary Source

Electricity can't be pumped out of the ground like oil or captured from moving air like wind energy. So it is called a secondary source of energy, meaning that it is produced by the use of primary energy sources such as coal, natural gas, or nuclear reactions. Electricity plays such an essential role in contemporary America that its supply and demand are often examined separately from the primary sources used to produce it.

Experts predict a 35% increase in demand for electricity by 2030.18 In practical terms, that means an equivalent increase in demand for coal and gas, at least for the next decade: Electricity generating plants now consume two-fifths of U.S. energy from all sources, including 90% of America's coal and nearly 30% of its natural gas. 19

There is no immediate way to alter that situation. In the near term, renewable sources such as solar, wind, and geothermal are unlikely to substantially change the mix of our energy supply. (And integrating the energy from many of these renewable energy sources would likely require substantial expansion of the electric transmission system.) While nuclear generation is a zero-atmospheric-emissions alternative that already produces one-fifth of America's electricity, efforts to increase that capacity face two large, though not insurmountable, hurdles: high capital investment costs and resistance from citizens groups that oppose the use and storage of radioactive material.

Getting electric power to consumers may be as much of a problem as generating it. Generating stations usually are built away from load centers because sites are easier to find and fewer people are disturbed by the accompanying noise, emissions, and activity. This power must be delivered by a high-voltage transmission system that has become increasingly stressed in recent years as growing demand has outstripped capacity. Widespread blackouts are possible, as evidenced by the August 2003 disruption to 50 million customers from Ohio to New York and Canada. New transmission lines are difficult to build because of uncertain cost recovery and public opposition. Building small plants near customers, known as distributed generation, may become more important in order to meet demand and maintain reliability.

Energy sources used to generate electricity in the United States in 2006.

Energy sources used to generate electricity in the United States in 2006.*20
* Percentages do not sum to 100% due to independent rounding.

So the basic question remains: How long can we maintain our petroleum dependency? The EIA cites known conventional oil reserves at more than 1.3 trillion barrels worldwide,21 and the U.S. Geological Survey estimates that there may be another 600 billion barrels undiscovered to date.

At present, total world consumption is approximately 85 MBD, 21 million of which is used by the United States.22 The nation's dependency on oil and the rapidly rising demand for oil in other countries, such as China and India, are heightening concern that we will reach a point where the oil supply can no longer be increased to meet projected demand. While this will certainly be true eventually, there is no consensus as to whether we are already entering that period or it is decades away. Pinning down an exact time frame is nearly impossible as estimates of the amount of “recoverable” oil available can change depending on new discoveries, technological developments, and price.

Over the past century, dependence on vehicles burning petroleum-based fuels has become a defining component of American life, bringing countless benefits. However, combustion of gasoline and diesel fuel emits carbon dioxide, as well as particulate matter, oxides of nitrogen (a prime component of “smog”), carbon monoxide, and unburned hydrocarbons. Indeed, whenever any fossil fuels are burned, carbon dioxide is released into the atmosphere, where it functions as a heat-trapping greenhouse gas.

Efforts are already well under way to find suitable alternatives to oil. In the short term, the leading liquid substitute is ethanol (“grain alcohol”), now chiefly made from corn. The federal government has an aggressive program to encourage its production. As a result, in 2005 about 4 billion gallons of fuel ethanol mixed with gasoline hit the domestic market.23 But in the same year, the United States consumed about 140 billion gallons of gasoline and 40 billion gallons of diesel fuel, so ethanol accounted for only a small percentage of the total gasoline pool.24

Ethanol raises other concerns. One drawback of corn ethanol production is that it requires a large amount of land and fresh water, along with inputs of fertilizers and energy. This results in potential competition with food sources for land use and fresh water for other industrial and commercial uses. In addition, with current technology, two-thirds of the energy value of corn ethanol is used just to produce the fuel – and most of that energy comes from fossil-fuel-based electricity or heating, offsetting much of the benefit.25

Natural Gas

Unlike oil, our natural gas comes primarily from North America. The annual volume of consumption is projected to rise from 21.8 trillion cubic feet (TCF) in 2006 to about 23.4 TCF in 2030.26 New activity in Alaska will supply some of that, but most will likely come from the lower 48 states and the Gulf of Mexico. Although the nation imports less than 3% of its natural gas from outside North America,27 it is forecast that imports will increase in the next few decades, from 0.5 TCF per year in 2006 to 2.9 TCF per year in 2030.28 These imports will largely take the form of liquefied natural gas, which is natural gas cooled to its liquid phase, making it easier to transport.

Global consumption of natural gas in 2004 was 100 TCF.29 Known world reserves of conventional natural gas total about 6,000 TCF, with perhaps another one-tenth of that amount still undiscovered.30 At that rate, known reserves will be adequate for about 60 years.

Natural gas is often described as “clean burning” because it produces fewer undesirable by-products than gasoline. Like all fossil fuels, its combustion emits carbon dioxide, but at about half the rate of coal.


America has plenty of coal. Its mines produced 1.2 billion tons in 2006, nearly all of it destined for electricity generation.31 That was a record year, but it barely scratched the surface of U.S. recoverable coal reserves, which are estimated at about 270 billion tons.32 More than one-fourth of the total known world coal reserves are in the United States, and supplies are sufficient for hundreds of years at current consumption rates.

Demand is projected to increase by 30% between now and 2030, propelled by rising use of electricity and possibly the expanded use of still-developing technology that converts coal to liquid fuel.33 Most of the increased supply will probably come from western states, which now provide about six-tenths of the nation's coal. Wyoming alone accounted for 38% of all domestic coal mined in 2006.34

Of all the fossil-fuel sources, coal is the least expensive for its energy content. In 2005, a million BTUs of energy from coal cost approximately $2, compared to $5 for natural gas and $10 for petroleum.35 However, burning coal in electric power plants is a major source of CO2 emissions, and its use has repercussions beyond combustion. Mining coal disturbs the land and modifies the chemistry of rainwater runoff, which in turn affects stream and river water quality. Coal-fired power plant emissions include oxides of nitrogen, sulfur dioxide, particulate matter, and heavy metals (such as mercury) that affect air quality and human health, often even hundreds of miles from the power plant. In response to strict environmental laws, “clean coal technologies” are being developed to reduce harmful emissions and improve the efficiency of these plants.

Renewable Energy Sources

Use of renewable energy sources will increase, in some cases dramatically, over the next two decades. While they may make significant contributions to the energy supply in certain geographic areas, absent major changes in economic, political, or technological factors, they will still provide a small fraction of our overall energy.

Hydropower is unlikely to increase much between now and 2030, but energy from biomass products (which include wood and wood byproducts, municipal waste, methane from landfills, and fuel from agricultural crops) will likely increase more than 60% by 2030.36 Energy from wind, solar, and other renewable sources is expected to nearly triple. But the net effect of all that activity will probably only raise the total contribution of renewables from 7% of total consumption now to about 8% in 2030.37

Hydroelectric production currently accounts for about 2.9% of our total energy production, while geothermal accounts for about 0.4%.38 Wind and solar-to-electric technologies account for a very small part of our total energy production, but wind, currently assisted by a production tax credit, has been penetrating the market rapidly in the past few years and accounted for almost 1% of the electricity generated in the United States in 2006.

The idea of drawing our energy from sources that are renewable, are independent of foreign nations, and do not emit greenhouse gases has powerful appeal. But capturing these resources is expensive, and many are intermittent, or sporadic, which complicates using them on a large scale. Further development promises reduced costs and improved storage and controls to overcome the intermittency problem.

Nuclear Fuel

America is unlikely to face problems in obtaining enough uranium ore to meet anticipated demand for several decades. According to government estimates, output from nuclear power plants is expected to increase only 18% by 2030.39 However, a U.S. nuclear renaissance is possible, and a growing number of nuclear plant design and construction permits have been submitted to the Nuclear Regulatory Commission over the past year. Some countries have successfully embraced nuclear power generation: for example, nuclear power plants produce nearly 80% of all electricity in France. In the United States, the issue prompts considerable debate, including concern over security and arguments about where and how to dispose of nuclear waste. But interest is growing, and nuclear energy may one day play a much larger role in supplying America's electricity.

Even with renewed U.S. interest in nuclear power generation, sufficient uranium supplies will likely be available. According to the Council on Foreign Relations, known worldwide reserves are adequate for about 70 years at current consumption rates and under current policies.40

The Flow of Energy

This figure depicts the flow of energy, measured in quadrillion (1 million billion) BTUs, across the energy system of the United States for 2006, based on data from the Energy Information Administration of the U.S. Department of Energy. The chart illustrates the connections between primary energy resources (fossil, nuclear, and renewables), shown at the far left, and end-use sectors categorized into residential, commercial, industrial, and the three principal components of transportation: cars, freight, and aviation. Electricity, a carrier derived from primary resources, powers the sectors to varying degrees and is positioned closer to the middle of the chart to display its inputs and outputs. Note that hydro, wind, and solar electricity inputs are expressed using fossil-fuel plants' heat rate to more easily account for differences between the conversion efficiency of renewables and the fuel utilization for combustion- and nuclear-driven systems. This enables hydro, wind, and solar to be counted on a similar basis as coal, natural gas, and oil. For this reason, the sum of the inputs for electricity differs slightly from the displayed total electricity output.

The Flow of Energy

(Click to Enlarge)

Source: LLNL 2008; data is based on DOE/EIA-0384(2006), June 2007. If this information or a reproduction of it is used, credit must be given to the Lawrence Livermore National Laboratory and the Department of Energy, under whose auspices the work was performed. Distributed electricity represents only retail electricity sales and does not include small amounts of electricity imports or self-generation. Energy flows for non-thermal sources (i.e., hydro, wind, and solar) represent electricity generated from those sources. Electricity generation, transmission, and distribution losses include fuel and thermal energy inputs for electric generation and an estimated 9% transmission and distribution loss, as well as electricity consumed at power plants. Total lost energy includes these losses as well as losses based on estimates of end-use efficiency, including 80% efficiency for residential, commercial, and industrial sectors, 20% efficiency for light-duty vehicles, and 25% efficiency for aircraft


Getting More for Less

Given the anticipated growth in every U.S. economic sector and in demand for all energy sources, it's natural to wonder how that growth can possibly be sustained. After all, America, with only 5% of the planet's population, already consumes one-fifth of the world's total energy. And other countries are poised to experience increases in energy use as they become more industrialized and improve their standard of living. Can the United States actually meet its growing needs?

It remains to be seen. Yet one important factor is working in our nation's favor. The demand for energy has not been growing as rapidly as the economy, resulting in a significant drop in what is called energy intensity. At present, Americans use about half as much energy per dollar of Gross Domestic Product (GDP) – the total market value of all the goods and services produced in a country during one year – as they did in 1970. Were it not for this development, the U.S. energy bill would be hundreds of billions of dollars per year higher. Energy-efficiency investments and structural shifts in the economy away from energy-intensive industry and toward service and information-based jobs have both contributed to the phenomenon. So have engineering improvements in scores of systems, from automobile engines to building insulation to electric power-generating facilities.

This trend is expected to continue. The EIA projects that by 2030 Americans will be using only slightly more energy per capita than they did in 1980 – but less than half as much per dollar of GDP.

Energy use per capita

Energy use per capita and per dollar of GDP from 1980 to 2030.41

Continuing this downward trend in energy intensity depends in part on the nation taking advantage of numerous opportunities for efficiency advances in current technology. Fortunately, recent history provides ample evidence that efficiency research and education can pay enormous dividends.

Next: Improving Efficiency →


Introduction | Sources and Uses | Supply and Demand | Improving Efficiency | Emerging Technologies | Looking Ahead | References and Credits

© 2008 by the National Academy of Sciences. All rights reserved.