"In the calorie department, corn is king. In 2014, average yield in the United States was 171 bushels per acre. ... Each bushel weighs 56 pounds and each pound of corn yields about 1,566 calories. That means corn averages roughly 15 million calories per acre. ....If you had taken our 2014 corn harvest of 14.2 billion bushels and used it to feed people, it would have met 17 percent of the entire world’s caloric needs.
By contrast, wheat comes in at about 4 million calories per acre, soy at 6 million. Rice is also very high-yielding, at 11 million, and potatoes are one of the few crops that can rival corn: They also yield about 15 million ....Other vegetables, while much more nutritious than corn, wheat or potatoes, are far less energy-dense. Broccoli yields about 2.5 million calories per acre, and spinach is under 2 million."
Each person needs about 1 million calories per year.