Conversion from Terajoules to Megajoules

Information about the Terajoule (TJ)

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.

Information about the Megajoule (MJ)

The megajoule is a unit of energy equal to one million joules. Its symbol is MJ. It is the scale at which energy stops being a laboratory quantity and becomes something with visible physical consequences.

A moving vehicle is the easiest anchor. A family car of 1500 kilograms travelling at 100 kilometres per hour carries about 0.58 megajoules of kinetic energy, and all of it must go somewhere in a collision. Crash structures are designed around exactly this figure, which is why impact energy rather than speed is the quantity written into vehicle safety standards.

Explosives are rated here too. A stick of dynamite releases roughly one megajoule, and a kilogram of TNT about 4.2. Because the TNT equivalent is defined rather than measured, the megajoule provides the bridge between chemical explosives and other energy releases, allowing a meteor impact or an industrial accident to be described on the same scale as a demolition charge.

Daily human energy intake is around ten megajoules, which is the same figure as 2400 kilocalories seen from the other side. Physical work is comparable: a day of manual labour above resting metabolism costs a few megajoules, and a long cycling stage costs perhaps fifteen.

Fuel energy densities live at this scale per unit mass. Petrol holds about 46 megajoules per kilogram, wood around 16, and a modern lithium battery about 0.9. That last comparison, a factor of fifty, is the single number that explains why electric aircraft remain difficult while electric cars do not.

Industrial and military systems use the unit for stored and delivered energy. Railgun and electromagnetic launcher performance is quoted in megajoules at the muzzle, flywheel storage in megajoules, and the energy delivered by a large capacitor bank in a fusion experiment likewise.

Comparing storage technologies in this unit is instructive because the gaps are so wide. A kilogram of petrol holds about 46 megajoules, a kilogram of dry wood about 16, and a kilogram of lithium-ion battery only about 0.9 — some fifty times less than the fuel. That single ratio explains most of the engineering difficulty of electric aviation and much of the weight of an electric car. Working the other way, a kilogram of uranium-235 fissioned completely releases about 80 million megajoules, and a kilogram of matter converted entirely to energy would give 90 billion. The unit spans that whole range without changing prefix more than a few times.

One megajoule equals one million joules, about 0.278 kilowatt-hours, or about 239 kilocalories.