Energy converter

The joule, the SI unit of energy, is the reference point for everything on this page. A kilojoule is 1,000 J, a calorie (thermochemical) is 4.184 J, and a kilowatt-hour — the unit on your electricity bill — is 3,600,000 J, since it's defined as one kilowatt of power sustained for one hour.

The default pairing is kilowatt-hour to megajoule. Multiply by 3.6 to go from kilowatt-hour to megajoule:

MJ = kWh × 3.6

Reverse it by multiplying by 0.2777777778, the same as dividing by 3.6. A household using 30 kWh in a day consumes 30 × 3.6 = 108 MJ of energy, and a food label listing 250 kcal represents 250 × 4.184 = 1,046 kJ.

Each corner of this category belongs to a different industry by habit more than necessity. Utility bills and renewable-energy capacity are quoted in kilowatt-hours and megawatt-hours. Nutrition labels use kilocalories, almost always just printed as “Calories.” Natural gas bills in the US use therms and BTU. Physics and chemistry at the atomic scale switch to electronvolts, since a joule is an absurdly large unit for the energy of a single particle collision. Geologists and seismologists even borrow “tons of TNT” as an intuitive way to compare the destructive energy of earthquakes and explosions to a familiar reference.

The single most common mix-up in this category isn't between two units on this list — it's that the “Calorie” printed on food packaging (capital C) is actually a kilocalorie, 1,000 times the small calorie (lowercase c) used in chemistry. A food described as “200 calories” really contains 200 kilocalories, or about 836.8 kJ. Separately, the calorie itself comes in two slightly different flavors — the thermochemical calorie (4.184 J) and the International Table calorie (4.1868 J) — a difference small enough to ignore for cooking but worth knowing about if you're reconciling scientific data from different sources.