Conversion from Gigajoules to Hectojoules

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 Hectojoule (hJ)

The hectojoule is a unit of energy equal to one hundred joules. Its symbol is hJ. Like the decajoule below it, it is a properly formed metric unit that hardly anybody writes, because a three-digit number of joules is already short enough and because the kilojoule waits only one decimal place further up.

The quantity is squarely human. Lifting a ten-kilogram case by one metre takes about 98 joules, so almost exactly one hectojoule. Climbing a single stair step costs an adult roughly 120 joules, and a hundred-watt lamp burns one hectojoule every second. A resting adult body releases about one hectojoule of heat per second simply by staying alive, which makes the unit a fair measure of one second of ordinary existence.

Several familiar quantities land here without carrying the name. A studio flash is rated between one and ten hectojoules, though the trade calls them watt-seconds. A defibrillator delivers two hectojoules in a shock. Structural steel is required to absorb at least 27 joules in a notched-bar impact test, and tough grades absorb well over one hectojoule, which is the difference between a plate that bends and one that shatters.

The prefix itself is far from dead, but it survives attached to other units. The hectopascal is the unit of atmospheric pressure in every weather forecast on earth, chosen precisely because one hectopascal equals one millibar exactly and forecasters could adopt SI without changing a single number on their charts. The hectare measures land almost everywhere, and the hectolitre is the trading unit for wine, beer and milk.

With the joule, none of those conditions were met. There was no legacy unit for the hectojoule to match, no established range of values it made tidier, and no trade that needed it. Engineering had already fixed on kilo as the first step up, so the hectojoule was left describing a range that ordinary decimal notation handles perfectly well on its own.

Reading one is therefore a matter of a single multiplication. A hundred hectojoules is ten kilojoules; four hectojoules is 400 joules. The joule form is what modern specifications, laboratory reports and datasheets will use, so converting on sight keeps a figure consistent with everything printed around it.

One hectojoule equals 100 joules, about 23.9 calories, or about 0.0278 watt-hours.