megajoule

The joule is the standard unit of energy in the international system of units. NIST lists it among the SI derived units, defined as a newton metre: the energy transferred when a force of one newton moves something one metre. The prefix mega means a million, so a megajoule is a million joules.

Energy and power are easy to confuse and the distinction is worth holding. A joule is an amount of energy. A watt is a rate, one joule per second. So a megajoule says how much energy was involved in something, while a wattage says how fast it is being delivered. The same amount of energy released quickly or slowly gives the same joule figure and very different effects.

Megajoules appear in fusion reporting because a single experiment is a burst rather than a continuous output. The National Ignition Facility's landmark shot delivered 2.05 megajoules of laser energy to its target and released 3.15 megajoules of fusion energy, while the facility drew something like 300 to 400 megajoules from the electrical grid to fire those lasers. Those three numbers, all in the same unit, are what make the comparison between them meaningful, and are why the unit is worth recognising.