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.
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.