Conversion from Kilojoules to Watt-hours

Information about the Kilojoule (kJ)

The kilojoule is a unit of energy equal to 1000 joules. Its symbol is kJ. It is the working unit of nutrition labelling in most of the world outside North America, and the natural scale for chemical reactions, mechanical impacts and small quantities of stored energy.

Food labelling is where most people meet it. European Union regulations require energy to be declared in kilojoules with kilocalories alongside, and Australia and New Zealand use kilojoules alone. A slice of bread holds roughly 300 kilojoules, a banana about 400, and daily requirements run from about 8000 to 10,500. The figures are larger than the kilocalorie equivalents by a factor of 4.184, which is why the two scales are rarely confused once seen together.

Chemistry adopted it wholesale. Enthalpies of reaction, bond dissociation energies, lattice energies and activation barriers are tabulated in kilojoules per mole, and the numbers land conveniently: a typical single covalent bond takes between 150 and 500 kilojoules per mole to break, and the combustion of methane releases 890.

Mechanical impacts sit at the same scale. A car at 50 kilometres per hour carries around 150 kilojoules of kinetic energy per tonne, a rifle bullet a few, and a boxer's punch about one. Protective equipment standards specify the energy a helmet or a barrier must absorb in kilojoules, which is what makes the ratings comparable across designs.

The unit describes stored energy in devices too small for kilowatt-hours. A camera flash capacitor holds a few kilojoules, a defibrillator discharge is around 0.2, and a compressed-air tool cycle uses a handful. Physiology uses it for the energy cost of movement, where a flight of stairs costs a person around 20 kilojoules.

Its main competitor for public use is the kilocalorie, which persists in the United States and in casual speech nearly everywhere. Where both appear, the kilojoule is the one that connects to the rest of physics without a conversion factor.

Materials testing uses the unit in a distinctive way. The Charpy test drops a weighted pendulum onto a notched bar and reports the energy absorbed in breaking it, typically a few tens of joules for steel at room temperature but falling sharply as the temperature drops. Plotting that energy against temperature reveals the ductile-to-brittle transition, the point at which a metal stops bending and starts shattering. The investigation of the Liberty ships that split in half in cold Atlantic water during the Second World War rested on exactly this measurement, and modern steel specifications still state a minimum absorbed energy at a stated temperature.

One kilojoule equals 1000 joules, about 239 calories, or about 0.278 watt-hours.

Information about the Watt-hour (Wh)

The watt-hour is a unit of energy equal to the energy delivered by one watt of power flowing for one hour, or exactly 3600 joules. Its symbol is Wh. It is the small change of the kilowatt-hour, used wherever quantities of energy are too modest to state in thousands.

Batteries are its main territory. The capacity of a laptop, phone, power tool or portable speaker is quoted in watt-hours, and this is the figure that matters when comparing devices, because it combines the two numbers usually printed on a cell. A battery marked 3000 milliampere-hours at 3.7 volts stores about 11 watt-hours, and the ampere-hour figure alone is meaningless without the voltage.

Aviation regulation has made the unit unexpectedly public. Lithium batteries above 100 watt-hours may not be carried in aircraft cabin baggage without airline approval, and above 160 watt-hours they are banned from passenger aircraft entirely. This is why laptop and camera batteries carry the figure printed on them, and why portable power banks are designed to sit just under the threshold.

Small-scale energy generation uses it as well. A solar panel on a garden light, an energy-harvesting sensor or a bicycle dynamo produces watt-hours per day rather than kilowatt-hours, and low-power electronics designed to run for years on a single cell are budgeted in milliwatt-hours.

The unit shares the criticism levelled at the kilowatt-hour: it combines a coherent SI unit of power with a non-SI unit of time. In strictly SI terms the correct expression is 3600 joules, or 3.6 kilojoules, and scientific writing uses joules. Everywhere else the practical link to a device's power rating wins.

Energy storage systems, from a home battery to a grid installation, scale upward through kilowatt-hours and megawatt-hours from the same starting point, so the whole family shares one arithmetic.

A common confusion is worth clearing up here. Battery cells are usually labelled in ampere-hours or milliampere-hours, which measure charge and not energy, and two cells with the same ampere-hour rating store quite different amounts if their voltages differ. Multiplying ampere-hours by the nominal voltage gives watt-hours: a 3000-milliampere-hour phone cell at 3.85 volts holds about 11.6 watt-hours. That is why capacity comparisons between devices are only meaningful in watt-hours, why airlines set their limits in watt-hours, and why a power bank advertised in milliampere-hours may deliver less than its number suggests once its own conversion losses are counted.

One watt-hour equals 3600 joules, 0.001 kilowatt-hours, or about 0.86 kilocalories.