Conversion from 2 Watt-hours to Megajoules

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.

Information about the Megajoule (MJ)

The megajoule is a unit of energy equal to one million joules. Its symbol is MJ. It is the scale at which energy stops being a laboratory quantity and becomes something with visible physical consequences.

A moving vehicle is the easiest anchor. A family car of 1500 kilograms travelling at 100 kilometres per hour carries about 0.58 megajoules of kinetic energy, and all of it must go somewhere in a collision. Crash structures are designed around exactly this figure, which is why impact energy rather than speed is the quantity written into vehicle safety standards.

Explosives are rated here too. A stick of dynamite releases roughly one megajoule, and a kilogram of TNT about 4.2. Because the TNT equivalent is defined rather than measured, the megajoule provides the bridge between chemical explosives and other energy releases, allowing a meteor impact or an industrial accident to be described on the same scale as a demolition charge.

Daily human energy intake is around ten megajoules, which is the same figure as 2400 kilocalories seen from the other side. Physical work is comparable: a day of manual labour above resting metabolism costs a few megajoules, and a long cycling stage costs perhaps fifteen.

Fuel energy densities live at this scale per unit mass. Petrol holds about 46 megajoules per kilogram, wood around 16, and a modern lithium battery about 0.9. That last comparison, a factor of fifty, is the single number that explains why electric aircraft remain difficult while electric cars do not.

Industrial and military systems use the unit for stored and delivered energy. Railgun and electromagnetic launcher performance is quoted in megajoules at the muzzle, flywheel storage in megajoules, and the energy delivered by a large capacitor bank in a fusion experiment likewise.

Comparing storage technologies in this unit is instructive because the gaps are so wide. A kilogram of petrol holds about 46 megajoules, a kilogram of dry wood about 16, and a kilogram of lithium-ion battery only about 0.9 — some fifty times less than the fuel. That single ratio explains most of the engineering difficulty of electric aviation and much of the weight of an electric car. Working the other way, a kilogram of uranium-235 fissioned completely releases about 80 million megajoules, and a kilogram of matter converted entirely to energy would give 90 billion. The unit spans that whole range without changing prefix more than a few times.

One megajoule equals one million joules, about 0.278 kilowatt-hours, or about 239 kilocalories.