Joule (J)
Definition: The SI unit of energy, work, or heat.
History: Named after English physicist James Prescott Joule.
Current Use: Standard unit for energy in scientific applications.
Kilojoule (kJ)
Definition: A unit of energy equal to 1000 joules.
History: Commonly used for food energy content.
Current Use: Used for nutritional information and larger energy measurements.
Megajoule (MJ)
Definition: A unit of energy equal to one million joules.
History: Used for large energy measurements.
Current Use: Used for vehicle fuel efficiency and large energy systems.
Gigajoule (GJ)
Definition: A unit of energy equal to one billion joules.
History: Used for very large energy measurements.
Current Use: Used for industrial energy consumption and power plants.
Calorie (cal)
Definition: A unit of energy, originally defined as the energy needed to raise 1g of water by 1°C.
History: Defined by French chemist Nicolas Clément.
Current Use: Used in nutrition and food science.
KiloCalorie (kcal)
Definition: A unit of energy equal to 1,000 gram calories, often used to express the energy content of food.
History: Also known as the "large calorie" or "food calorie," it was historically used to measure the energy content of food before the widespread adoption of the joule.
Current Use: The most common unit for expressing the energy value of food and is what people typically refer to when they say "calorie" in a dietary context.
Electron Volt (eV)
Definition: A unit of energy equal to the kinetic energy gained by an electron accelerated through 1 volt.
History: Fundamental unit in atomic and particle physics.
Current Use: Used in atomic, nuclear, and particle physics.
Therm (thm)
Definition: A unit of energy equal to 100,000 BTU.
History: Used in natural gas industry.
Current Use: Standard unit for natural gas billing.
Foot-pound (ft·lb)
Definition: A unit of energy equal to the work done by one pound-force over one foot.
History: Traditional imperial unit.
Current Use: Used in the US for mechanical work and energy.
BTU per hour (BTU/h)
Definition: A unit of power equal to 0.293071 watts, used for measuring heat transfer rates.
History: Derived from the British thermal unit (Btu) and the hour, commonly used in heating and cooling systems.
Current Use: Used in the HVAC industry to express heating and cooling capacities.
British Thermal Unit (BTU)
Definition: A unit of energy equal to about 1055 joules.
History: Traditional British unit for heat energy.
Current Use: Used in heating, ventilation, and air conditioning.
Calorie (15°C) (cal₁₅)
Definition: The amount of heat to raise 1g of water from 14.5°C to 15.5°C.
History: Used in thermochemistry.
Current Use: Rare, but found in older scientific literature.
Calorie (International Table) (cal_IT)
Definition: Defined as exactly 4.1868 joules.
History: Used in international tables for energy conversion.
Current Use: Rare, but found in older scientific literature.
Calorie (Nutritional) (Cal)
Definition: The dietary calorie, equal to 1 kilocalorie.
History: Used in food energy labeling.
Current Use: Standard for food energy in many countries.
Calorie (Thermochemical) (cal_th)
Definition: Defined as exactly 4.184 joules.
History: Used in thermochemistry.
Current Use: Standard in thermochemical calculations.
Erg (erg)
Definition: A unit of energy in the CGS system, equal to 10^-7 joules.
History: From Greek "ergon" meaning work.
Current Use: Used in physics and astronomy for small energies.
Gram Calorie (cal)
Definition: The amount of heat needed to raise 1g of water by 1°C.
History: Used in chemistry and nutrition.
Current Use: Still used in some scientific contexts.