Energy has more everyday units than almost any other quantity. Physicists use the joule, named after James Prescott Joule, whose experiments in the 1840s showed that heat and mechanical work are the same thing. Food labels use kilocalories, written kcal or Calories with a capital C, where one kcal is about 4.184 kilojoules. And energy bills use kilowatt-hours: one kWh is exactly 3.6 million joules, the energy used by a 1-kilowatt appliance running for an hour.
How your gas bill turns cubic metres into kWh
Gas meters measure volume, but suppliers bill by energy. On a UK bill, the volume used is converted to kilowatt-hours using a standard formula: the volume in cubic metres, multiplied by a correction factor of 1.02264 for temperature and pressure, multiplied by the calorific value of the gas, usually around 39 to 40 megajoules per cubic metre, then divided by 3.6 to turn megajoules into kWh. Using 100 cubic metres and a calorific value of 39.5, that's 100 × 1.02264 × 39.5 ÷ 3.6, or about 1,122 kWh. Older imperial meters measure in hundreds of cubic feet, which must be converted to cubic metres first by multiplying by 2.83. If a bill looks wrong, checking which type of meter you have is a good first step.
Precision and pitfalls
The most frequent mistake is confusing energy with power. A kilowatt is a rate, and a kilowatt-hour is an amount. A 3 kW kettle running for three minutes uses 0.15 kWh. A 7 kW EV charger running for ten hours delivers 70 kWh, minus some losses. The second mistake is mixing small and large calories: a "200 calorie" snack is 200 kcal, about 837 kJ, not 200 joules' worth of calories.
Worked conversions
A typical adult daily food intake of 2,000 kcal is about 8.4 megajoules, or roughly 2.3 kWh. That's less energy than a single long hot bath heated electrically, which helps explain why heating water is one of the biggest parts of many households' energy use. At an assumed electricity price of 25p per kWh, the 0.15 kWh kettle boil costs under 4p, while a 70 kWh EV charge at home costs around £17.50. The calculator above handles all of these units, including British thermal units, which still appear on some boiler and air-conditioning specifications.
Comparing fuels
Energy units make it possible to compare very different fuels on the same basis. A litre of petrol contains roughly 9 kWh of energy, and a litre of diesel slightly more, around 10 kWh. A typical electric car might use 25 to 30 kWh to travel 100 miles, while a petrol car doing 45 miles per UK gallon burns about 10 litres over the same distance, roughly 90 kWh of fuel energy, about three times as much. Most of the difference is lost as heat in the engine, which is why electric motors are so much more efficient in energy terms, even before counting how the electricity was generated.
Checking your answer
Remember a few anchors. One kWh is 3.6 MJ. One kilocalorie is about 4.2 kJ. A typical home in Great Britain uses around 2,700 kWh of electricity and 11,500 kWh of gas a year under Ofgem's typical consumption values. If a converted figure says a single appliance uses more energy in a day than a whole house does in a year, look again at whether watts and watt-hours have been confused.
One more worked example
Boiling exactly one litre of water from 15°C needs about 0.1 kWh in theory, because it takes 4.18 kJ to raise a kilogram of water by one degree, and 85 degrees × 4.18 kJ is about 355 kJ. Real kettles use a little more, because some heat warms the kettle itself and escapes. Filling the kettle with only the water you need is one of the simplest ways to cut that waste.