Btu (15°C)
The Btu (15°C) is a specific variant of Btu to reflect the energy needed to raise the temperature of water at 15 degrees Celsius by one degree.
Joules
The joule (J) is the SI unit of energy, named after the English physicist James Prescott Joule. It is the work done when a force of one newton moves an object by one meter in the direction of the force, so it is sometimes known as a newton-meter. It is also one watt-second.