I am ashamed at the number of things around my house and shops that are done by … human beings. . . . Hereafter a motor must do all the chores.

– Thomas Edison, “The Tomorrows of Electricity and Invention,” 1910

Labor Day should also be celebrated as Energy Appreciation Day. Why? Because energy, defined as the capacity to do work, has been instrumental in America’s productivity gains, wealth creation, and increased leisure time.

For almost all of human history, work was limited by a person’s muscles or those of domesticated animals. This changed with the Industrial Revolution where massive amounts of dense, mineral energies powered the machines of progress. “Fossil fuels cannot explain the start of the industrial revolution,” Matt Ridley explains in The Rational Optimist. “But they do explain why it did not end.”

Living a life of opulence once took hundreds of servants or slaves. Nearly 500 prepared King Louis XIV’s royal banquets at Versailles in 1700. Everything was done by hand from the slaughterhouse to the table, and without delay. Today, with refrigeration and truck delivery, much more is at the ready in countless varieties.

“Inanimate energy slaves” (IES) became part of the nomenclature in 1940 when Buckminster Fuller estimated the human-equivalent of energy in useful work. For the world’s two billion, he calculated 17 IES per person, with the U.S. having several times more. “Mechanization, the harnessing of energy,” he wrote, “is man’s answer to slavery.”

Jennifer Barker 65 years later calculated that the average American had 149 IES working around the clock. Driving a car added one thousand more, given that a fit person was about one-tenth of a horsepower.

Fossil fuels, the batteries of the sun, are at the center of humankind’s leverage over the limits of nature. Going backwards to the dilute, intermittent flow of the sun would reintroduce a life of toil. Depletion of mineral energies was feared for this reason, but oil, natural gas, and coal are expansive resources with discoveries and technology opening up new frontiers of supply.

The transition from human and animal power to machine power has made energy the master resource. Erich Zimmermann in the mid-twentieth century noted:

The shift to machine power changed America from a rural agricultural nation to an industrial giant. It also made men’s lives easier and richer. In 1850, the average American worked seventy hours a week. Today he works forty-three. In 1850, our average American produced about 27 cents’ worth of goods in an hour. Today he produces about $1.40 worth in dollars of the same purchasing power.

“By providing energy flows of high power density, fossil fuels and electricity made it possible to embark on a large-scale industrialization,” Vaclav Smil more recently observed, “creating a predominantly urban civilization with unprecedented levels of economic growth reflected in better health, greater social opportunities, higher disposable incomes, expanded transportation and an overwhelming flow of information.”

The benefits of modern energy have allowed labor to take days off to relax and celebrate—and even created the opportunity to permanently retire from work. This Labor Day, we should take time to appreciate the master resource in its many manifestations.