BTU (International) (BTU)
The British Thermal Unit (BTU), symbolized as BTU, is an imperial unit of energy used to measure heat. One BTU is the heat needed to raise one pound of water by 1°F. The BTU originated in the 19th century, during the Industrial Revolution, for heating in the UK and the US. It is still used in HVAC systems and energy production. The BTU connects imperial and metric energy systems, especially in countries that use imperial units for industry.
Calorie (cal)
The calorie (cal) originated in the 19th century as a unit for quantifying heat energy. It was initially defined as the energy required to increase the temperature of one gram of water by one degree Celsius under standard atmospheric pressure. The calorie gained popularity in fields such as chemistry, physics, and nutrition. Despite not being an official SI unit, it remains prevalent on nutritional labels and in dietary contexts, particularly in the United States and Europe. Before the widespread adoption of the joule, the calorie played a significant role in standardizing energy measurements related to food and biological systems.