| Reference | Joules (J) | Terajoules (TJ) |
|---|---|---|
| One food calorie (kcal) | 4184 J | 0.000000004184 TJ |
| An AA alkaline battery | 10000 J | 0.00000001 TJ |
| Daily adult food intake | 8368000 J | 0.000008368 TJ |
| One unit on an electricity bill | 3600000 J | 0.0000036 TJ |
| A lightning strike | 1 × 109 J | 0.001 TJ |
The joule is the SI unit of energy, work and heat. Its symbol is J. One joule is the work done when a force of one newton moves its point of application through one metre, and equally the energy dissipated when one watt of power flows for one second.
That equivalence is the reason the unit exists. Before the nineteenth century, heat and mechanical work were treated as different things measured in different units, heat in calories and work in foot-pounds. James Prescott Joule spent years demonstrating that a fixed amount of mechanical work always produces the same amount of heat, most famously by stirring water with a paddle wheel driven by falling weights and measuring the temperature rise. Establishing that exchange rate, the mechanical equivalent of heat, unified mechanics and thermodynamics into one subject.
Because it is defined through the base units, the joule connects everything. It is a newton metre, a watt second, a coulomb volt and a kilogram metre squared per second squared, and every one of those forms appears in ordinary use depending on which branch of physics is speaking. Electrical, mechanical, thermal and chemical energy are all counted in the same unit, which is what makes energy accounting across a system possible at all.
The scale is human-sized in a useful way. Lifting an apple one metre takes about one joule. A heartbeat uses roughly one joule. Dropping a textbook from a desk releases about ten. Below that, joules become clumsy and the prefixed units take over; above it, kilojoules and megajoules do the same.
Food energy is properly measured in joules, and labels in Australia, New Zealand and the European Union give kilojoules alongside or instead of calories. The calorie survives in general use mainly through habit, and the conversion, one calorie equal to 4.184 joules, is fixed by definition rather than measured.
Nutrition, ballistics, battery capacity, laser output, explosive yield and the energy released by an earthquake are all quoted in joules or its multiples, which allows quantities from completely different domains to be compared directly.
One joule equals one newton metre, one watt second, about 0.239 calories, or about 0.000278 watt-hours.
The terajoule is a unit of energy equal to a thousand gigajoules, or 1,000,000,000,000 joules. Its symbol is TJ. It is the unit in which countries account for their energy, and it appears far more often in official statistics than in any engineering catalogue, because it is sized for the annual consumption of a town rather than of a machine.
International energy accounting runs on it. The International Energy Agency publishes national energy balances in terajoules, and greenhouse-gas inventories submitted under the United Nations climate framework report fuel use in the same unit. Emission factors are expressed to match: burning a terajoule of natural gas releases about 56 tonnes of carbon dioxide, diesel about 74, and hard coal about 95, which turns any energy figure directly into an emissions figure.
Generation is described here too. A three-megawatt wind turbine running at a realistic capacity factor produces roughly 33 terajoules in a year. A single cargo of liquefied natural gas in a large carrier holds close to 3800 terajoules, which is why one shipment can cover a small country's gas demand for weeks. Grid-scale batteries, by contrast, are still measured in fractions of a terajoule.
Destructive energy reaches the scale as well. A kilotonne of TNT equals 4.184 terajoules exactly, so the bomb dropped on Hiroshima released roughly 63 terajoules. An earthquake of magnitude five radiates about two terajoules of seismic energy, and each whole step up the magnitude scale multiplies that by about thirty-two, which is what makes a magnitude seven event a thousand times the size of a magnitude five one.
The unit also marks a boundary in how energy is discussed. Below it, quantities belong to buildings, vehicles and appliances, and people argue about efficiency. At and above it, quantities belong to policy: fuel imports, subsidy schemes, emission targets and grid planning. The same physics is being measured, but the terajoule is where the conversation stops being technical and becomes political.
Above the terajoule the ladder continues with the petajoule, used for whole national sectors, and the exajoule, used for world totals: global primary energy consumption runs to something over 600 exajoules a year. Reading any of these back down is a matter of moving the decimal point three places at a time.
One terajoule equals 1,000,000,000,000 joules, about 277.8 megawatt-hours, or about 239 gigacalories.