The metric unit of heat energy is the calorie (heat can also be measured in joules, the metric unit of energy in general). It's the amount of energy that must be input (i.e., turned into entropy) to raise the temperature of one gram of water by one degree Celsius.

It has achieved fame as the measure of energy derivable when an animal eats a given amount and type of food.

I once looked at a British candy bar wrapper, and my eyes bugged out. 300 kilocalories??? Then I remembered.

When "Calorie" is capitalized, it refers to one metric kilocalorie, the amount of energy needed to rase one kilogram of water by one degree Celsius, or one gram of water by one thousand degrees Celsius, which doesn't actually happen.

American nutritional labels use Calories, regardless of whether the designer remembered to capitalize it. Trust me.