Energy has one honest unit, the joule, and a collection of historical ones that persist because their industries never let go. Food uses calories, electricity uses kilowatt-hours, heating uses BTUs, and physics uses joules. All of them measure the same quantity.
Quick reference
The table follows whichever units you select above.
The calorie problem
There are two calories, differing by a factor of a thousand, and both are written with the same word.
The small calorie is the energy needed to heat one gram of water by one degree Celsius — 4.184 J. The large Calorie, properly capitalised, is a kilocalorie: 4,184 J. Nutrition labels use the large one. A 500-Calorie meal is 500 kcal, or about 2.1 million joules.
Most of the world's food labels sidestep this by printing kilojoules instead, which is why an Australian or European label shows numbers roughly four times larger than an American one for the same food. Nothing differs but the unit.
| Item | Energy | Also |
|---|---|---|
| Apple | 95 kcal | 398 kJ |
| Daily adult intake | ~2,000 kcal | ~8,400 kJ |
| 1 kWh of electricity | 860 kcal | 3.6 MJ |
Kilowatt-hours are energy, kilowatts are power
This is the distinction electricity bills depend on. A watt is a rate — a joule per second. A kilowatt-hour is what you get by sustaining one kilowatt for an hour, which is 3.6 million joules.
So a 2 kW heater running for three hours uses 6 kWh. Saying an appliance "uses 2 kWh" without a time period is like describing a journey as "60 miles per hour" without saying how long you drove.
BTUs on air conditioners are usually per hour. A unit advertised as "12,000 BTU" almost always means 12,000 BTU per hour, a rate of about 3.5 kW. The "per hour" gets dropped in marketing, turning a power rating into what looks like an energy one.
Common questions
How many kilojoules in a calorie?
A food Calorie (kcal) is 4.184 kJ. To convert a label from kcal to kJ, multiply by 4.184; going back, divide.
How many joules in a kWh?
Exactly 3,600,000. One kilowatt sustained for 3,600 seconds.
Why do European and American food labels look so different?
Because most of the world prints kilojoules and the US prints kilocalories. The kJ figure is about 4.2 times the kcal figure for identical food.