Kilojoules
A kilojoule (kJ) is equivalent to one thousand joules (J). It is the energy required when a force of one thousand newtons is applied over a distance of one meter, or one newton applied over a kilometer. In reality, a joule is roughly the energy required to lift a small apple one meter against Earth's gravity.
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.