megajoule

The megajoule is the unit of fuel and of motion at vehicle scale. A 1500 kg car at 100 km/h carries about 0.6 MJ of kinetic energy, which its crash structure must dissipate in a fraction of a second. A litre of gasoline holds about 34 MJ, so the full tank of a small car stores around 1700 MJ, which is why liquid fuel is so hard to displace: matching that store at today's pack energy densities takes roughly three tonnes of lithium battery. An adult human runs on about 10 MJ of food per day, an average of 115 watts, which is why a crowded room warms like a hall of old light bulbs.

Its most everyday appearance is on Australian gas bills, which charge by the megajoule. The meter records cubic metres, and the bill multiplies by the gas's measured heating value, around 38 MJ/m³, so households there buy energy directly rather than volume, a more honest basis than the volumetric billing used in much of the world since heating value varies with gas composition.

Fuel comparison tables in MJ per kilogram and MJ per litre are where energy policy arguments become arithmetic. Hydrogen is spectacular by mass at about 120 MJ/kg, nearly three times gasoline, and dismal by volume even when liquefied, a single pair of numbers that explains most of the difficulty of hydrogen aviation.