Btu (3.9°C)
The Btu (3.9°C) is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit at the temperature where water reaches its maximum density which occurs at approximately 3.9°C (39°F).
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.