Convert energy

About the units

• Joules (J) – The joule is the SI coherent unit of energy, work and heat, fixed by the General Conference on Weights and Measures: one joule is a kilogram times a square metre per second squared, the work done by a force of one newton over one metre. It is a legal unit throughout the EU and the natural one in physics, chemistry and engineering, while food labelling counts in kilojoules. An electronvolt is 1.602176634 × 10⁻¹⁹ joules. The trap is the newton metre: same dimension, but torque is not energy.
• Kilojoules (kJ) – A kilojoule is 1,000 joules, since the prefix kilo means 10³, and a legal unit throughout the EU. It is the compulsory energy unit in nutrition labelling: the EU food information regulation requires energy in kilojoules and kilocalories, both. The factors behind the figure sit in the same regulation – fat 37 kJ per gram, carbohydrate and protein 17, alcohol 29. One kilocalorie is 4.184 kilojoules. Because those factors are rounded separately, the two numbers on a pack do not convert exactly into one another.
• Megajoules (MJ) – A megajoule is a million joules, since the prefix mega means 10⁶, and as a decimal multiple of the joule a legal unit throughout the EU. It is used for the energy content of fuels, for technical energy balances and for raw food data. The simple anchor is that one kilowatt-hour is exactly 3.6 megajoules. Two traps: megajoules and megawatt-hours are a factor of 3,600 apart, and megajoules per kilogram is not megajoules per litre, since oil and diesel differ in density.
• Calories (cal) – A calorie is not one unit. The thermochemical calorie is exactly 4.184 joules and the international table calorie 4.1868 joules; the conversion here uses the thermochemical one. The calorie is absent from the EU units directive and so is not a legal unit of measurement, while the kilocalorie may appear on a pack alongside the kilojoule. The real trap is the name: food “calories” are in fact kilocalories, a thousand times larger. A banana of 100 calories holds 100,000 calories in the strict sense.
• Kilocalories (kcal) – A kilocalorie is 1,000 calories, exactly 4,184 joules on the thermochemical calorie this conversion is built on. It is the energy unit consumers actually read, but in the EU it may appear only together with the kilojoule: the food information regulation requires both. Nutrition figures are worked out from fixed factors – fat 9 kilocalories per gram, carbohydrate and protein 4, alcohol 7. American labels write “Calories” with a capital C, meaning the same thing. Kilocalorie against calorie is a factor of a thousand.
• Watt-hours (Wh) – A watt-hour is the energy of one watt for one hour, exactly 3,600 joules. As a compound of two legal units it is legal throughout the EU. Batteries are labelled in it, and air travel rules rest on it: lithium-ion batteries of up to 100 Wh may go in the cabin, those above 100 and up to 160 Wh need the airline’s approval, and spare batteries and power banks may never go in the hold. Milliamp-hours alone need the voltage: 20,000 mAh at 3.7 volts is 74 Wh.
• Kilowatt-hours (kWh) – A kilowatt-hour is the energy of one kilowatt for one hour, exactly 3.6 megajoules. It needs no definition of its own: the watt and the hour are both legal units in the EU, and combinations of legal units are legal too. It is the unit on the electricity bill and on the EU energy label, where a fridge’s annual consumption is given as a whole number of kilowatt-hours per year. The trap is kW against kWh: the kilowatt is power, the kilowatt-hour energy.
• Megawatt-hours (MWh) – A megawatt-hour is 1,000 kilowatt-hours, exactly 3.6 gigajoules. It is the unit of the wholesale electricity market: on the Nord Pool exchange, where power for the Nordic countries among others is traded a day ahead, prices are in euros per megawatt-hour, while household bills count kilowatt-hours. A price of 100 euros per megawatt-hour is therefore 10 euro cents per kilowatt-hour. Annual output needs bigger units still: the Swedish reactor Forsmark 3 produced about 8.8 terawatt-hours in 2025, 8.8 million megawatt-hours. The trap is megawatt against megawatt-hour, power against energy.
• British thermal units (BTU) – The British thermal unit comes in several variants up to 0.5% apart. This conversion uses the international table Btu, exactly 1,055.05585262 joules; the thermochemical variant is 0.067% smaller. The Btu is the general energy unit in the US – a gallon of petrol is around 120,000 Btu and a therm exactly 100,000 – but it is absent from the EU units directive. The commonest trap is air conditioning: “12,000 Btu” means Btu per hour, a power, and differs from the energy by a factor of 3,600.
• Foot-pounds (ft·lb) – A foot-pound is the work of one pound-force over one foot, exactly defined because the pound and the foot are exact: about 1.356 joules. Absent from the EU units directive, it survives in American and British engineering and firearms law. In England and Wales an air rifle counts as specially dangerous, needing a firearm certificate, if the pellet leaves the muzzle with more than 12 foot-pounds, and an air pistol above 6. That is 16.3 and 8.1 joules. The trap is word order: “ft-lb” is energy and “lb-ft” torque.
• Electronvolts (eV) – An electronvolt is the kinetic energy an electron gains in passing through a potential difference of one volt in a vacuum. Since the elementary charge was fixed in 2018, 1 eV is exactly 1.602176634 × 10⁻¹⁹ joules; the EEC units directive of 1971 still called its value not exactly known. With prefixes it runs through physics and medicine: an X-ray tube at 80 kilovolts produces photons of at most 80 keV, and CERN’s LHC has collided protons at 13.6 TeV. The trap: the electronvolt is energy, not voltage.