Units

calorie (cal)-the amount of heat required at one atmoshpere to raise one gram of water by one degree Celsius.

Calorie (kcal)-one thousand times the above quantity.

The latter is how your food is represented on nutritional labels.  The average adult requires 2000 Calories per day just to sustain itself.  This is equivalent to the amount of heat required to raise the temperature of a metric ton of water by one degree Celsius.  A seemingly enormous amount until you consider that this is the same energy required to power a 60-Watt light bulb for a day and a half.  Makes you appreciate the efficiency of the human body.