Energy unit conversion table
Each unit with its value in joules and its equivalent in the two units people most often convert to.
| Unit | Joules | In kJ | In kWh |
|---|---|---|---|
| 1 kcal | 4,184 | 4.184 | 0.0011622 |
| 1 kJ | 1,000 | 1 | 0.00027778 |
| 1 Wh | 3,600 | 3.6 | 0.001 |
| 1 kWh | 3,600,000 | 3,600 | 1 |
| 1 BTU | 1,055.0559 | 1.0550559 | 0.00029307 |
| 1 therm | 105,480,400 | 105,480.4 | 29.300111 |
- The calorie used here is the thermochemical calorie, exactly 4.184 J. The International Table calorie is 4.1868 J — a 0.07% difference. Thermochemical is the value behind food labelling: EU Regulation 1169/2011 converts with 1 kcal = 4.184 kJ.
- The BTU also has several definitions. This converter uses the International Table BTU (1,055.05585262 J); the thermochemical BTU is about 0.05% smaller. For everyday heating figures the difference is far below the accuracy of the input data.
- Energy is not power. A kilowatt is a rate; a kilowatt-hour is the energy delivered by that rate over an hour. Converting between them needs a duration — that is what the kWh calculator on this site does.
What are the common energy units?
Energy is the capacity to do work, and the SI unit is the joule (J) — one newton acting through one metre. A joule is small at human scale, so practical units are larger: the kilojoule (1,000 J) on European food labels, the watt-hour (3,600 J, one watt sustained for one hour) and the kilowatt-hour (3.6 million J) that electricity meters count.
The calorie causes more confusion than any other unit here, because two different things share the name. The thermochemical calorie is exactly 4.184 J. The Calorie on a nutrition label, written with a capital C or as kcal, is one thousand of those — 4,184 J. A 100 kcal snack therefore carries 418.4 kJ, which is why both numbers appear side by side on European packaging.
The British thermal unit is the customary heat unit in the United States and in the HVAC trade worldwide; the International Table BTU used here is 1,055.05585262 J. A therm is 100,000 BTU, the unit natural gas is billed in, and equals about 29.3 kWh. The foot-pound is the customary mechanical work unit, and the electronvolt — 1.602176634 × 10⁻¹⁹ J since the 2019 SI redefinition — is the natural unit of atomic and particle physics.
How to use this energy converter
- Enter the energy value you want to convert.
- Select its unit — kilocalories for food labels, kWh for electricity, therms or BTU for gas and heating.
- Read the equivalent in every other unit at once.
- For food labels, use kcal rather than cal: the Calorie printed on packaging is a kilocalorie, a thousand times larger than the scientific calorie.
The conversion factors behind the energy converter
The value is converted to joules using its unit's factor, then divided by each target unit's factor. All the factors below are exact definitions except the therm, which is exact by statute in the United States.
Worked example: 100 kcal = 100 × 4,184 = 418,400 J = 418.4 kJ. In kilowatt-hours: 418,400 ÷ 3,600,000 = 0.11622222 kWh. In BTU: 418,400 ÷ 1,055.05585262 = 396.56668 BTU.
Results are shown to eight significant figures rather than a fixed number of decimals, because these units span 27 orders of magnitude — one electronvolt is 0.00000000000000000016 J, and any fixed rounding would display it as zero.
Common mistakes
- Confusing the calorie with the food Calorie. A nutrition label reading 250 Calories means 250 kcal, which is 250,000 scientific calories or 1,046 kJ.
- Treating kW and kWh as the same quantity. A 2 kW heater running for 3 hours uses 6 kWh; the power never changed.
- Mixing BTU definitions in precision work. The International Table and thermochemical BTU differ by about 0.05%.
- Assuming a therm is a metric unit. It is 100,000 BTU, a US gas-billing unit, and equals about 29.3 kWh.
Часто задаваемые вопросы
How many kJ is 1 kcal?
Exactly 4.184 kJ. One kilocalorie is 4,184 joules by the thermochemical definition, which is the value EU food-labelling rules use, so a 100 kcal portion is 418.4 kJ. This is why European packaging prints both numbers.
Is a Calorie the same as a calorie?
No, and the difference is a factor of 1,000. The calorie (lowercase) is 4.184 J. The Calorie on a nutrition label — sometimes written with a capital C, more often as kcal — is a kilocalorie, 4,184 J. Food energy is always the larger one.
How many BTU are in a kWh?
3,412.1416 BTU. One kilowatt-hour is 3,600,000 J and one International Table BTU is 1,055.05585262 J, so 3,600,000 ÷ 1,055.05585262 = 3,412.1416. The rounded figure 3,412 is the one usually quoted in HVAC work.
How many kWh is a therm of gas?
29.300111 kWh. A US therm is 100,000 BTU, or 105,480,400 J, and a kilowatt-hour is 3,600,000 J. This conversion is what lets you compare a gas bill quoted in therms against an electricity bill quoted in kWh.
What is an electronvolt?
The energy an electron gains crossing a potential difference of one volt: exactly 1.602176634 × 10⁻¹⁹ J since the 2019 SI redefinition fixed the elementary charge. It is the natural unit for atomic, nuclear and particle physics, where joules would mean carrying nineteen leading zeros.
Источники
- NIST Special Publication 811 (Thompson & Taylor). Guide for the Use of the International System of Units (SI), 2008 — Appendix B energy conversion factors, including the thermochemical calorie and International Table BTU.
- Bureau International des Poids et Mesures (BIPM). The International System of Units (SI Brochure), 9th edition, 2019 — the joule, and the exact value of the electronvolt following the 2019 redefinition.
- Regulation (EU) No 1169/2011 on the provision of food information to consumers, Annex XIV — energy conversion factors used for nutrition declarations.