Btu
The British Thermal Unit (Btu) is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is primarily used in the United States to describe the energy content of fuels and the power of heating and cooling systems.
Calories (15°C,CIPM, 1950)
The calorie defined as CIPM, 1950 or the "15°C calorie" is a unit of energy that was historically used to quantify the amount of heat required to raise the temperature of one gram of water by one degree Celsius at 15°C. This definition was established by the International Committee for Weights and Measures (CIPM) in 1950.