kilojoule

The kilojoule is the food energy unit for most of the world. Australian and New Zealand labels state energy in kJ, and packaging there carries the reference figure of 8700 kJ, the benchmark adult daily intake that plays the role 2000 Calories plays in the United States. The two conventions describe the same physics through the exact factor 4.184: 8700 kJ is about 2080 kcal, and a label showing both is showing one measurement twice.

Away from food, the kilojoule is the working unit of thermodynamics tables, where specific enthalpies come in kJ/kg, and the water numbers are worth a feel. Heating a kilogram of water from 20 °C to boiling takes about 335 kJ; turning that boiling kilogram into steam takes a further 2257 kJ, nearly seven times as much. That ratio is the reason steam is how industry moves heat, and the reason a steam scald is so much worse than a splash of hot water at the same temperature: the condensing steam hands back all of that latent heat.

For body-scale intuition, a 70 kg person climbing one 3 metre flight of stairs does about 2 kJ of work against gravity, so the 8700 kJ on the label would notionally climb thousands of flights. Metabolism is far from perfectly efficient, but the gap between what food carries and what lifting costs is the honest arithmetic behind why exercise struggles to outrun diet.