Energy Converters
Energy shows up on electricity bills as kilowatt-hours, on food labels as calories or kilojoules, and in heating specifications as BTUs. Each field adopted its own unit, so comparing them takes a conversion. The converters here use the standard definitions (for example, one thermochemical calorie is exactly 4.184 joules and one kilowatt-hour is exactly 3.6 million joules), so you can compare appliances, diets and fuel on the same footing.
All energy converters
Things to know about energy conversions
- The "Calorie" on a food label is a kilocalorie: 1,000 of the small calories used in physics, equal to 4,184 joules.
- Electricity is billed in kilowatt-hours. One kWh equals 3.6 million joules, or about 3,412 BTU.
- BTU ratings on heaters and air conditioners usually refer to BTU per hour, which is a power rating, not an amount of energy.
Energy conversion chart
Read across: one of the unit in the left column equals the value shown under each unit.
| 1 unit = | J | kJ | cal | kcal | Wh | kWh | BTU |
|---|---|---|---|---|---|---|---|
| J | 1 | 0.001 | 0.239 | 0.000239 | 0.0002778 | 2.778 × 10-7 | 0.0009478 |
| kJ | 1,000 | 1 | 239 | 0.239 | 0.2778 | 0.0002778 | 0.9478 |
| cal | 4.184 | 0.004184 | 1 | 0.001 | 0.001162 | 1.162 × 10-6 | 0.003966 |
| kcal | 4,184 | 4.184 | 1,000 | 1 | 1.162 | 0.001162 | 3.966 |
| Wh | 3,600 | 3.6 | 860.4 | 0.8604 | 1 | 0.001 | 3.412 |
| kWh | 3,600,000 | 3,600 | 860,421 | 860.4 | 1,000 | 1 | 3,412 |
| BTU | 1,055 | 1.055 | 252.2 | 0.2522 | 0.2931 | 0.0002931 | 1 |
Energy units explained
- Joule (J)
- The joule is the SI unit of energy: the work done by a force of one newton over one meter. A small apple lifted one meter takes about one joule.
- Kilojoule (kJ)
- A kilojoule is 1,000 joules. It is used for food energy on labels in Australia, New Zealand and Europe, usually alongside kilocalories.
- Calorie (small) (cal)
- The small (thermochemical) calorie is exactly 4.184 joules, the energy needed to warm one gram of water by one degree Celsius.
- Kilocalorie (food Calorie) (kcal)
- A kilocalorie is 1,000 small calories, or 4,184 joules. The "Calories" on food labels are actually kilocalories.
- Watt-hour (Wh)
- A watt-hour is the energy used by a one-watt device in an hour, exactly 3,600 joules. Battery capacities are often given in Wh.
- Kilowatt-hour (kWh)
- A kilowatt-hour is the energy used by a 1,000-watt device in one hour, exactly 3.6 million joules. It is the unit electricity is billed in.
- British thermal unit (BTU)
- One BTU (International Table) is about 1,055 joules, the heat needed to raise one pound of water by one degree Fahrenheit. It is used to rate air conditioners and furnaces.