Calories (I.T.)
The Calorie (I.T.) or International Table Calorie is defined as the amount of energy required to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere which is approximately 4.1868 joules.
Btu (15.6°C)
The Btu (15.6°C) specifically refers to the energy needed to raise the temperature of water from 60°F (15.6°C) to 61°F.