| Nanojoules (nJ) | Terajoules (TJ) |
|---|---|
| 1 Nanojoule | 1 × 10-21 TJ |
| 2 Nanojoules | 2 × 10-21 TJ |
| 3 Nanojoules | 3 × 10-21 TJ |
| 4 Nanojoules | 4 × 10-21 TJ |
| 5 Nanojoules | 5 × 10-21 TJ |
| 10 Nanojoules | 1 × 10-20 TJ |
| 20 Nanojoules | 2 × 10-20 TJ |
| 25 Nanojoules | 2.5 × 10-20 TJ |
| 50 Nanojoules | 5 × 10-20 TJ |
| 100 Nanojoules | 1 × 10-19 TJ |
| Reference | Nanojoules (nJ) | Terajoules (TJ) |
|---|---|---|
| One food calorie (kcal) | 4.184 × 1012 nJ | 0.000000004184 TJ |
| An AA alkaline battery | 1 × 1013 nJ | 0.00000001 TJ |
| Daily adult food intake | 8.368 × 1015 nJ | 0.000008368 TJ |
| One unit on an electricity bill | 3.6 × 1015 nJ | 0.0000036 TJ |
| A lightning strike | 1 × 1018 nJ | 0.001 TJ |
The nanojoule is a unit of energy equal to one billionth of a joule, or 0.000000001 joules. Its symbol is nJ. It marks the point where mechanics gives way to optics and electronics: far too little to move anything the eye can see, yet still very large compared with the energy carried by a single particle. Almost every quantity written in nanojoules comes off an instrument rather than out of everyday experience.
Light gives the clearest picture of the scale. A photon of green light carries about 3.6 × 10⁻¹⁹ joules, so a nanojoule is roughly 2.8 billion such photons arriving together. Measured against heat it is larger still: the average thermal energy of a single molecule at room temperature is about 4 × 10⁻²¹ joules, which makes one nanojoule some 240 billion times that. The unit therefore sits well above the quantum world and well below the mechanical one.
Pulsed lasers are where it earns its keep. The oscillator in a two-photon microscope typically delivers pulses of a few nanojoules at eighty million pulses a second, which averages out to a few hundred milliwatts of beam power. Fibre lasers, optical coherence tomography scanners and time-of-flight rangefinders are all specified the same way, because what damages a sample or returns a usable echo is the energy in each pulse, not the average power spread across the second.
Digital electronics is measured against the nanojoule from below. A single switching event inside a processor costs femtojoules, a memory access a few picojoules, so one nanojoule pays for hundreds of memory reads or millions of logic operations. Energy-harvesting design inverts the picture: an indoor photovoltaic cell, a piezoelectric pickup or a radio-frequency antenna may gather only a few nanojoules per second, and the whole craft of ultra-low-power engineering consists of fitting useful work inside that budget.
Ordinary objects reach the scale only when they are very small or barely moved. A grain of sand weighing a milligram, falling a tenth of a millimetre, releases about one nanojoule. Lifting a single human hair by its own width costs less than that. Nothing in a kitchen or a workshop is ever priced in nanojoules, which is precisely why the unit belongs to laboratories, laser catalogues and datasheets rather than to bills and labels.
In writing, the prefix competes with scientific notation. Physics papers usually print 10⁻⁹ J and move on, while instrument manufacturers print nJ because it fits on a specification line and reads without arithmetic. Both express the same quantity, and the choice says more about the intended reader than about the measurement. Where a table mixes pulse energies spanning several decades, the prefixed forms are generally easier to compare at a glance.
One nanojoule equals 0.000000001 joules, about 6.24 × 10⁹ electronvolts, or about 2.78 × 10⁻¹³ 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.