"To survive, Weaver said, humans have a single basic need: "usable energy." That energy comes in two forms: energy for the body (food and water, in other words), and energy for daily existence (that is, fuel to power vehicles, heat and cool buildings, and make essential materials like cement and steel). "In the United States," Weaver estimated, "each person uses, on the average, 3,000 calories per day for food, [and] 125,000 calories per day for heat and power."