| Decajoules (daJ) | Terajoules (TJ) |
|---|---|
| 1 Decajoule | 1 × 10-11 TJ |
| 2 Decajoules | 2 × 10-11 TJ |
| 3 Decajoules | 3 × 10-11 TJ |
| 4 Decajoules | 4 × 10-11 TJ |
| 5 Decajoules | 5 × 10-11 TJ |
| 10 Decajoules | 1 × 10-10 TJ |
| 20 Decajoules | 2 × 10-10 TJ |
| 25 Decajoules | 2.5 × 10-10 TJ |
| 50 Decajoules | 5 × 10-10 TJ |
| 100 Decajoules | 0.000000001 TJ |
| Reference | Decajoules (daJ) | Terajoules (TJ) |
|---|---|---|
| One food calorie (kcal) | 418.4 daJ | 0.000000004184 TJ |
| An AA alkaline battery | 1000 daJ | 0.00000001 TJ |
| Daily adult food intake | 836800 daJ | 0.000008368 TJ |
| One unit on an electricity bill | 360000 daJ | 0.0000036 TJ |
| A lightning strike | 100000000 daJ | 0.001 TJ |
The decajoule is a unit of energy equal to ten joules. Its symbol is daJ, and the spelling dekajoule with a k is also accepted. It is the least written of all the joule multiples, not because the quantity is obscure but because deca is the least used prefix in the entire metric system, and understanding that is more useful than memorising the unit itself.
The quantity is one of the easiest in physics to picture. Lifting a one-kilogram bag of sugar by one metre takes 9.81 joules, so almost exactly one decajoule. A heart beats about a joule per stroke, so ten beats. A rifle pellet from an unlicensed air rifle in Britain carries up to 16 joules, or a little over 1.6 decajoules. These are quantities the body understands directly as effort.
Deca survives in ordinary life in only a handful of places, and almost all of them involve mass. In Austria, Poland, Hungary and the Czech lands, delicatessen counters still sell by the dekagram, so a customer asks for twenty deka of ham rather than two hundred grams. The decanewton is used for rope and cable ratings and, in France, for towing capacities, because it happens to be close to the weight of a kilogram.
Elsewhere the prefix simply lost. Engineering standardised on steps of a thousand, which places kilo directly above the base unit and leaves nothing for deca to do that a two-digit number does not already do better. Writing 40 joules is shorter than writing 4 decajoules, and it sorts correctly in a table alongside 400 joules and 4 kilojoules without any mental arithmetic.
The scale itself is worth knowing even under another name, because it is the scale of single human actions. Throwing a ball, closing a heavy door, lifting a full kettle onto a hob, drawing a bow: each of these costs a few tens of joules. Impact tests on packaging and protective equipment are specified here too, and so are the energy limits written into rules for airguns and industrial fastening tools.
When a decajoule appears in a document, converting it takes one step in either direction. Multiply by ten for joules, divide by a hundred for kilojoules. Both forms are exact, and the joule form is the one that will match every other figure in a modern specification.
One decajoule equals 10 joules, about 2.39 calories, or about 0.00278 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.