Calorie - The calorie is a pre-SI metric unit of energy. The unit was first defined by Professor Nicolas Clement in 1824 as a unit of heat. This definition entered French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule. However, in many countries it remains in common use as a unit of food energy. In the context of nutrition, and especially food labelling, the terms calorie and kilocalorie are interchangeable. In either case the unit is approximately equal to 4.2 kJ.
Historically, the calorie has had two major alternative definitions differing by a factor of one thousand. In addition to these two major alternative definitions, minor variants of the definition of this unit also exist differing in the exact experimental conditions used, most notably the start temperature of the water.
Kilogram and gram calories - The original definition by Clement was based on the kilogram. Other definitions based on the gram have since been made. We thus have the two major variants: the kilogram calorie and the gram calorie. One thousand gram calories equal one kilogram calorie. In the context of food energy the term calorie generally refers to the kilogram calorie. However, the term kilocalorie (kcal), referring to one thousand gram calories, is also in widespread use especially by professional nutritionists (when speaking in terms of calories rather than joules). To avoid confusion, the prefix kilo- is not used with the kilogram calorie.
Kilogram calorie - The kilogram calorie, large calorie, food calorie, Calorie (capital C) or just calorie (lowercase c) is the amount of energy required to raise the temperature of one kilogram of water by one degree Celsius. Gram calorie - The gram calorie, small calorie or calorie (cal) is the amount of energy required to raise the temperature of one gram of water by 1 °C. The gram calorie was once commonly used in chemistry and physics.
BTU - The British thermal unit (BTU or Btu) is a traditional unit of energy equal to about 1.06 kilojoules. It is used in the power, steam generation, heating and air conditioning industries. In scientific contexts the BTU has largely been replaced by the SI unit of energy, the joule (J), though it may be used as a measure of agricultural energy production (BTU/kg). It is still used 'unofficially' in metric English-speaking countries (such as Canada, the United Kingdom), and remains the standard unit of classification for air conditioning units manufactured and sold in many non-English-speaking metric countries.
In North America, the term "BTU" is used to describe the heat value (energy content) of fuels, and also to describe the power of heating and cooling systems, such as furnaces, stoves, barbecue grills, and air conditioners. When used as a unit of power, BTU 'per hour' (BTU/h) is understood, though this is often confusingly abbreviated to just "BTU".
The unit MBTU was defined as one thousand BTU presumably from the Roman numeral system where "M" stands for one thousand (1,000). This is easily confused with the SI mega (M) prefix, which adds a factor of one million (1,000,000). To avoid confusion many companies and engineers use MMBTU to represent one million BTU. Alternatively a therm is used representing 100,000 or 105 BTU, and a quad as 1015 BTU.
Erg - An erg is the unit of energy and mechanical work in the centimetre-gram-second (CGS) system of units, symbol "erg". Its name is derived from the Greek ergon meaning work. An erg is the amount of work done by a force of one dyne exerted for a distance of one centimetre. In the CGS base units, it is equal to one gram centimetre-squared per second squared (g-cm2/s2). It is thus equal to 1 x 10-7 joules or 100 nanojoules (nJ) in SI units. 1 erg = 10-7 joule. 1 erg = 624.15 GeV = 6.2415 x 1011 eV. 1 erg = 1 dyn cm.
Joule - The joule (symbol J) is the derived unit of energy in the International System of Units. It is the energy exerted by a force of one newton acting to move an object through a distance of one metre. One joule is the amount of energy required to perform the following physical actions: The work done by a force of one newton traveling through a distance of one metre; The work required to move an electric charge of one coulomb through an electrical potential difference of one volt; or one coulomb volt, with the symbol C-V; The work done to produce the power of one watt continuously for one second; or one watt second (compare kilowatt hour), with the symbol W-s.
Kilowatt Hour - The kilowatt hour, also written kilowatt-hour (symbol kW-h, kW h or kWh) is a unit of energy equal to 3.6 megajoules. Energy delivered by electric utilities is usually expressed and charged for in kilowatt hours. Energy in kilowatt hours is the product of power in kilowatts and time in hours; it is not "kilowatts per hour".
The SI (International System of Units) unit of energy is the joule (J), equal to one watt second (one watt is equal to one joule per second); one kilowatt hour is exactly 3.6 megajoules, which is the amount of energy expended (or dissipated) if work is done at a constant rate of one thousand watts for one hour.
Other energy-related units - The kilowatt hour is a convenient unit for electrical bills because the energy usage of a typical electrical customer in one month is several hundred kilowatt hours. Megawatt hours, gigawatt hours, and terawatt hours are used for metering larger amounts of electrical energy.
The energy content of a battery is usually expressed indirectly as its capacity in ampere hours; to convert watt hours (W-h) to ampere hour (A-h), the watt hour value must be divided by the voltage of the power source. This value is approximate since the voltage is not constant during discharge.
Average annual power production or consumption can be expressed in kilowatt hours per year; for example, when comparing the energy efficiency of household appliances whose power consumption varies with time or the season of the year, or the energy produced by a distributed power source. One kilowatt hour per year is about 114.08 milliwatts.
The Board of Trade unit or B.O.T.U. is an obsolete UK synonym for kilowatt hour. The term derives from the name of the Board of Trade that regulated the electricity industry. The B.O.T.U. should not be confused with the British thermal unit or BTU, which is a much smaller quantity of thermal energy. To further the confusion, at least as late as 1937, Board of Trade unit was simply abbreviated "B.T.U." or "BTU."
Burnup of nuclear fuel is normally quoted in megawatt-days per ton (MWd/MTU), where ton refers to a metric ton of uranium metal or its equivalent, and megawatt refers to the entire thermal output, not the fraction which is converted to electricity.
Nutritional Calorie - Like other forms of energy, food energy is expressed in calories or joules. Some countries use the food calorie, which is equal to 1 kilocalorie (kcal), or 1,000 gram calories. In the context of nutrition, and especially food labeling, the calories are large calories approximately equal to 4.1868 kilojoules (kJ). The kilojoule is the unit officially recommended by the World Health Organization and other international organizations. In some countries only the kilojoule is normally used on food packaging, but the calorie is still the most common unit in many countries.
Only carbohydrates (including fiber), fats, proteins, organic acids, polyols, and ethanol contain food energy. All foods are made up of a combination of these six nutrients. Everything else in food is non-caloric, including (but not limited to) water, vitamins, minerals, antioxidants, caffeine, spices and natural flavors. Tea and coffee also have no calories without sugar or milk added. Nutritionists usually talk about the number of calories in a gram of a nutrient. Fats and ethanol have the greatest amount of food energy per gram, 9 and 7 kcal/g (38 and 30 kJ/g), respectively. Proteins and most carbohydrates have about 4 kcal/g (17 kJ/g). Carbohydrates that are not easily absorbed, such as fiber or lactose in lactose-intolerant individuals, contribute less food energy. Polyols (including sugar alcohols) and organic acids have fewer than 4 kcal/g.
Each food item has a specific metabolizable energy intake (MEI). Normally this value is obtained by multiplying the total amount of energy contained in a food item by 85%, which is the typical amount of energy actually obtained by a human after the digestive processes have been completed.