Conversion from Gigajoules to Joules

Information about the Gigajoule (GJ)

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.

Information about the Joule (J)

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.