Before technology and prosperity, virtually everyone spent long hard days scraping together enough calories for themselves and their family to survive.
If I am trying to portion my meals better, I could ask it, through a verbal interface, to vibrate gently when I reach seven hundred calories in that meal.
If you are a farmer and work alone, you can only plant as much land as you can personally plow. You can do just a couple of thousand calories of work a day, consuming only the energy produced by the food you ate.
The ox consumes about six thousand calories of energy, so now your combined output is four light bulbs' worth of energy. But if you get a diesel tractor (and presumably diesel fuel), the amount of energy you can consume goes up exponentially, and with that, the amount of work you can do.
How would you define calories? Add your definition here.