| Gigajoules (GJ) | Microjoules (µJ) |
|---|---|
| 1 Gigajoule | 1 × 1015 µJ |
| 2 Gigajoules | 2 × 1015 µJ |
| 3 Gigajoules | 3 × 1015 µJ |
| 4 Gigajoules | 4 × 1015 µJ |
| 5 Gigajoules | 5 × 1015 µJ |
| 10 Gigajoules | 1 × 1016 µJ |
| 20 Gigajoules | 2 × 1016 µJ |
| 25 Gigajoules | 2.5 × 1016 µJ |
| 50 Gigajoules | 5 × 1016 µJ |
| 100 Gigajoules | 1 × 1017 µJ |
| Reference | Gigajoules (GJ) | Microjoules (µJ) |
|---|---|---|
| One food calorie (kcal) | 0.000004184 GJ | 4.184 × 109 µJ |
| An AA alkaline battery | 0.00001 GJ | 1 × 1010 µJ |
| Daily adult food intake | 0.008368 GJ | 8.368 × 1012 µJ |
| One unit on an electricity bill | 0.0036 GJ | 3.6 × 1012 µJ |
| A lightning strike | 1 GJ | 1 × 1015 µJ |
The gigajoule is a unit of energy equal to a billion joules, or 1000 megajoules. Its symbol is GJ. Unlike the smaller prefixed joules it is a working commercial unit: gas and heat are bought and sold in gigajoules, and a household's annual energy use is a number of gigajoules that fits comfortably on one line of a bill.
Heating is where most people meet it. District heating in Denmark and the Netherlands is metered and invoiced in gigajoules, and Canadian natural gas bills are denominated in them. One gigajoule is roughly the heat in 26 cubic metres of natural gas, or in 29 litres of petrol, and a well-insulated house in a cold climate consumes something between 40 and 100 gigajoules a year for space heating and hot water.
The unit also measures what materials cost to make. Producing a tonne of structural steel takes roughly 20 to 25 gigajoules of primary energy, a tonne of cement about 4, and a tonne of primary aluminium close to 200. Those figures are why aluminium is described as stored electricity, and why embodied-energy tables in construction are published in gigajoules per tonne rather than in any smaller unit.
For destructive energy it is the natural scale between the domestic and the catastrophic. A tonne of TNT releases 4.184 gigajoules by definition, and a lightning strike delivers on the order of one to five, though almost all of that goes into heating and shattering the air rather than into whatever it hits. A tonne of hard coal holds about 29 gigajoules of chemical energy.
Human beings appear in the same table. An adult eating about ten megajoules a day consumes roughly 3.7 gigajoules of food energy a year, which is less than a single small car burns in a month. Setting the two figures side by side in one unit is one of the clearest ways to show how much of modern energy use happens outside the body.
Above the gigajoule, national statistics move on to terajoules and petajoules; below it, appliances and vehicles are described in megajoules and kilowatt-hours. The gigajoule sits at the join, which is why energy regulators and utilities converge on it: it is large enough for a building and small enough for a bill.
One gigajoule equals 1,000,000,000 joules, about 277.8 kilowatt-hours, or about 239,000 kilocalories.
The microjoule is a unit of energy equal to one millionth of a joule, or 0.000001 joules. Its symbol is µJ. It is the scale of small living things, of short laser pulses and of the electricity a sensor spends waking up, and it is the smallest energy unit that still describes events an unaided person can notice happening.
Insects live here. A flea of half a milligram leaping twenty centimetres spends about one microjoule, and a mosquito in level flight carries roughly a quarter of one. Lifting a gram by a tenth of a millimetre costs the same. These are the quantities that make the unit intuitive: not invisible in principle, merely too small to matter unless something repeats them millions of times a second.
Ignition safety is written in this range, and the numbers decide how equipment may be built. The minimum energy that will ignite a hydrogen and air mixture is about 17 microjoules; for most hydrocarbon vapours it is a few hundred. Intrinsically safe apparatus for refineries, grain stores and mines is certified by proving that no fault can release more than the mixture requires, which is why circuits destined for hazardous areas are audited in microjoules rather than in volts.
Pulsed light instruments are specified in microjoules almost universally. A vehicle lidar fires pulses of a few microjoules many thousand times a second, laser marking systems use tens to hundreds, and femtosecond amplifiers destined for micromachining sit at the top of the range. The energy per pulse sets what the beam can cut, image or safely illuminate, and the average power alone does not answer that question.
Low-power electronics budgets in the same terms. A passive radio-frequency identification tag harvests a few microjoules from the reader's field and must complete its whole exchange inside that. A short-range radio advertisement costs tens of microjoules, a quartz watch movement about one per second, and a temperature sensor that wakes, measures and sleeps again may spend under ten. Battery life on such devices is arithmetic performed in microjoules.
Above it the millijoule takes over and the effects become mechanical: a spark you feel, a click you hear, a small object dropped. Below it the nanojoule takes over and the effects become optical and electronic. The microjoule occupies the narrow band between, which is why so many disciplines that otherwise share nothing converge on the same prefix.
One microjoule equals 0.000001 joules, about 6.24 × 10¹² electronvolts, or about 2.78 × 10⁻¹⁰ watt-hours.