References and footnotes:
 Leslie A. White, The Evolution of Culture (New York: McGraw-Hill, 1959), 3.
 Leslie A. White, “Energy and the Evolution of Culture,” American Anthropologist 45, no. 3 (July-September, 1943): 335.
 Leslie A. White, Energy and the Evolution of Culture (New York: Grove Press, 1949), 111.
 For comparison, the dashed line ❹ represents the middle series forecast but with zero immigration. Used as a point of reference, it shows that about two thirds of the U.S. population growth through 2100 will be due to immigration.
 The reader should consider the implications of liberalized U.S. immigration policy, as proposed by some, on any estimate of the size of the U.S. population in 2100. Most immigrants come to America to “adopt” our standard of living which, by White’s Law, means they and their children are adding to our future energy needs. There is nothing in White’s Law granting them a waiver with respect to their impact on future U.S. energy needs.
 A British Thermal Unit or BTU is the amount of thermal energy required to increase the temperature of one pound of water by 1°F. The BTU was defined in the early days of steam engine development to quantify how much thermal energy was released by the combustion of fuels such as wood and coal. To understand better how much heat is involved, heating a cup of tap water to the start of boiling to make a cup of tea requires about 70 BTU.
 Note that annual energy production/consumption data reporting did not start until 1950. Prior to that year, reporting was at 5-year intervals creating the impression of less year-to-year variation.
 Beginning in the late 1950s, the United States began to import large quantities of oil as demand outpaced domestic production. In 1970, domestic oil production peaked even as domestic demand continued to grow. At this point, the U.S. vulnerability to a disruption in oil imports became significant as oil imports surged from about 1 billion BOE in 1970 to over 2 billion BOE in 1973 at the time of the first oil supply crisis.
 The first oil supply crisis arose in 1973 during the 4th Arab-Israeli War, also known as the Yom Kippur War. Due to a reversal of fortunes on the battlefield by the attacking Arab forces, some oil-exporting countries in the region initiated an embargo of the United States in an attempt to dissuade U.S. military support for Israel during the conflict. World oil prices more than doubled. While the military aspects of the conflict were resolved in fairly short order, the economic consequences persisted in the United States for nearly five years before per capita energy use returned to pre-crisis levels. The second oil supply crisis started following the hostage-taking of U.S. citizens in Iran in 1979. The hostage situation persisted for well over a year. In response, the United States embargoed oil imports from Iran. This drove world oil prices to near $100/barrel in 2010 dollars. With oil supplies constrained, with natural gas supplies also constrained due to over-regulation by the government, and with high world oil prices, the United States entered a severe recession with high unemployment, high interest rates, and high inflation. It took nearly a decade for per capita energy use, as a measure of the standard of living, to return to near pre-crisis levels.
 One important outcome of the second oil-supply crisis is that U.S. per capita oil consumption was permanently lowered—falling about 25%—despite oil prices returning, in the mid-1980s, to near pre-crisis levels. During the six years of the recession, the United States shifted away from oil where technologically and economically feasible. Coal production expanded to replace oil for electricity generation. Natural gas production, once it was deregulated, expanded to heat homes and supply industry. Nuclear electricity, in development since the 1950s, became commercially available to help meet growing demand for electricity. In all three cases, the costs of the replacement energy sources were less than the cost of the oil they replaced. The availability and affordability of these replacement energy sources enabled the United States to return to near pre-crisis per capita energy use as the 1980s ended. Note, however, that all of these substitution energy sources were also non-sustainable. Consequently, this was only a temporary fix.
 One unknown is the growth of humanoids—robots replacing humans at work or serving humans as machine butlers. It is possible there may be tens of millions of such robots in the United States in 2100, all requiring energy to operate, maintain, repair, replace, and transport.
 Currently, about two percent of the Earth’s land surface is peat bog. As the plants in these bogs die, they form the peat that begins the natural cycle for fossil fuel formation leading to coal. Peat accumulates at a rate of about 1 inch in 25 years. This illustrates that the natural cycle of fossil fuel formation continues even today, although at a very slow pace compared to humanity’s rate of extraction.
 Carl R. Behrens et al., “U.S. Fossil Fuel Resources: Terminology, Reporting, and Summary,” Congressional Research Service, R40872, December 28, 2011.
 The unit “watt” is named after James Watt, the 18th century inventor of the improved steam engine that enabled the industrial revolution.
 In these calculations, the contribution of renewables was included with that of nuclear-electricity since a hypothetical all-nuclear energy infrastructure is being assessed.
 Hydrogen, as a gas at normal pressure and temperature, has a density of only 0.006 lb/cu. ft. Thus, to store hydrogen in bulk, it must be compressed to high pressures. For comparison, natural gas storage is in the range of 2,000-4,000 psi when stored as a gas rather than a liquid. As it takes more energy to liquefy hydrogen, compared to pressurizing it to 6,500 psi, high pressure storage is the most likely method that would be used.
 The remaining 5% of the year—about 18 days—is used for refueling and plant maintenance. Modern plants operate up to 18 months between refueling.
 There are proposals for advanced fission nuclear power plants that use thermal energy to split water directly in the reactor to yield hydrogen. This is not, however, state-of-the-art for fission nuclear power.
 The condition under which any fuel is combusted controls how much useful thermal energy is produced. There are two standard sets of conditions for determining the useful thermal energy produced by gas and liquid fuels. These are referred to as the “lower heating value” or LHV and the “higher heating value” or HHV with the latter due to more efficient conditions of combustion such as ultra-high efficiency, combined-cycle gas turbines. Most other combustion conditions, such as home heating and transportation, fall in the LHV category. At the HHV conditions of hydrogen combustion, the author’s estimate is that 2,137 kWh of electricity is required per BOE of hydrogen compressed to 6,500 psi. Because the combustion process is more efficient, about 15% less electricity is needed to yield 1 BOE of net thermal energy. The LHV of hydrogen is 51,682 BTU/lb. Thus, 1 BOE equals 112.22 lb. of hydrogen or 50.9 kg. The author’s estimate of 2,529 kWh/BOE, for both electrolysis and compression to 6,500 psi for storage, corresponds to 50 kWh/kg. According to Wikipedia, the typical range today is 50-79 kWh/kg for just electrolysis. The author’s estimate anticipates some modest improvement in the efficiency of the electrolyzers and gas compressors.
 Feasibility Assessment of the Water Energy Resources of the United States for New Low Power and Small Hydro Classes of Hydroelectric Plants, DOE-ID-11263, January 2006, 1, http://hydropower.inl.gov/ resourceassessment/pdfs/main_report_appendix_a_final.pdf.
 United States Geological Survey Circular 790, Assessment of Geothermal Resources of the United States, 1978, http://www.geo-energy.org/aboutGE/potentialUse.asp.
 Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply, April 2005, http://www1.eere.energy.gov/biomass/pdfs/final_billionton_vision _report2.pdf
 James Michael Snead, “The End of Easy Energy and What to Do About It,” 2008, 82.
 The nameplate generation capacity of a panel is based on tests under simulated sunlight positioned directly over the panel. It is the maximum output of the panel under ideal conditions that rarely occur in practice.
 The available wind power is a function of the wind’s velocity raised to the third power. Hence, increasing the turbine’s hub height generally raises the rotor into winds of higher speed, making more wind power available to be harnessed. Commercial wind turbines currently fall into two groups: 80 m hub heights, with a nameplate generation capacity of 1.5 MW, and 100 m hub heights with a 2.5 MW capacity. A wind turbine only produces its nameplate power when the wind speed is equal to or greater than the turbine’s rated speed but less than the maximum permitted speed. For 2.5-MW turbines, this is usually in the range of 28-56 mph. Below the rated speed of 28 mph (12.5 meters/sec), the electrical power output is less than the nameplate power. Below about 7 mph, the turbine is stopped. Above 56 mph the turbine is also stopped to prevent structural damage. Most of the time, the wind speed is below the rated speed, which is why the capacity factor is less than 100%. In the best areas, the capacity factor is in the range of 35-40%.
 5 MW of installed nameplate power per sq. km—12.9 MW per sq. mi.—is the value used by the federal government to estimate the optimum spacing of wind turbines in wind farms. The actual value for a specific wind farm depends on a number of factors including average wind speeds, terrain, and hub heights.
 As seen in Fig. 22, the 936,000 sq. mi. value corresponds to a minimum capacity factor of 30%. While wind farms can be built in areas with a lower capacity factor, some argue that economically this does not make sense.
 England had already passed this point when the first English settlers arrived in America in the 1600s. Endless old-growth forests stretching to the horizon were a fantastic sight to them.
 The first primary use of oil was to distill kerosene to replace whale oil for lighting. Natural gas then became a second source for lighting.
 See the work of American geophysicist M. King Hubbard with respect to his publications in the 1950s forecasting the peak in U.S. oil production around 1970.
 A satellite in geostationary orbit will enter the Earth’s shadow for up to several hours at local midnight on and near the spring and fall equinoxes. This corresponds to the period of typical minimum power demand due to the time of year and the time of day. Ground receiving stations would use secondary power, using stored hydrogen, to generate electricity during this period. All ground receiving stations would have secondary power generators for peak power and emergency generation needs.
 The gross solar insolation on 1.7 sq. mi. in geostationary orbit is about 6 GW. The conversion of this to electrical power, the transmission of the power to the ground receiving site, and the conversion back into electrical power fed to the local utility grid yields 1 GW. The end-to-end efficiency is about 17%.
 Gerard K. O’Neill, The High Frontier: Human Colonies in Space (New York: Morrow, 1976).