How the Energy Converter Works
Energy is measured in different units depending on where you meet it. Food labels use kilocalories or kilojoules, your electricity bill uses kilowatt-hours, heating and air conditioning use BTU, and science uses joules. The Energy Converter changes an amount of energy from one unit into another, so you can compare them.
What you enter
- The value: a number, which can have decimals.
- The unit you have (From): joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours or BTU.
- The unit you want (To): any of the same units.
How the conversion works
Every unit has a fixed size in joules. The converter turns your energy into joules, and then turns the joules into the unit you chose:
joules = value x joules in the From unit result = joules / joules in the To unit
These are the sizes of the units:
| Unit | Size in joules |
|---|---|
| 1 joule (J) | 1 |
| 1 kilojoule (kJ) | 1,000 |
| 1 calorie (cal) | 4.184 |
| 1 kilocalorie (kcal) | 4,184 |
| 1 watt-hour (Wh) | 3,600 |
| 1 kilowatt-hour (kWh) | 3,600,000 |
| 1 BTU | 1,055.06 |
A kilocalorie is 1,000 calories. A kilowatt-hour is 1,000 watt-hours. A watt-hour is the energy of 1 watt used for 1 hour, which is 3,600 seconds, so it is 3,600 joules.
A worked example
A meal has 500 kilocalories (kcal). You want to know the energy in other units.
| Step | Value |
|---|---|
| Kilocalories to joules (500 x 4,184) | 2,092,000 J |
| Joules to kilojoules (2,092,000 / 1,000) | 2,092 kJ |
The same 500 kcal in other units:
| Unit | Amount |
|---|---|
| Joules | 2,092,000 J |
| Kilojoules | 2,092 kJ |
| Calories | 500,000 cal |
| Kilocalories | 500 kcal |
| Watt-hours | 581.11 Wh |
| Kilowatt-hours | 0.5811 kWh |
| BTU | 1,982.83 BTU |
One kilowatt-hour in other units
The kilowatt-hour is the unit on an electricity bill. One kWh is:
| Unit | Amount |
|---|---|
| Joules | 3,600,000 J |
| Kilojoules | 3,600 kJ |
| Kilocalories | 860.42 kcal |
| Watt-hours | 1,000 Wh |
| BTU | 3,412.13 BTU |
Food energy: kilocalories to kilojoules
| kcal | kJ |
|---|---|
| 100 | 418.4 |
| 200 | 836.8 |
| 500 | 2,092 |
| 1,000 | 4,184 |
| 2,000 | 8,368 |
Chart: food energy in kilojoules
The longer the bar, the more energy. The kilocalories are on the left, and the kilojoules on the right:
100 kcal ████ 418 kJ 200 kcal ████████ 837 kJ 500 kcal ████████████████████ 2,092 kJ 1,000 kcal ████████████████████████████████████████ 4,184 kJ
The kilojoule number is always about 4.2 times the kilocalorie number.
Chart: energy used by a device in a few hours
Energy is power multiplied by time. These are examples worked out from the power and the time, to show the size of a kilowatt-hour:
60 W bulb for 10 hours ██████ 0.6 kWh 100 W bulb for 10 hours ██████████ 1 kWh 1,500 W heater for 1 hour ███████████████ 1.5 kWh 2,000 W oven for 2 hours ████████████████████████████████████████ 4 kWh
Doing it by hand
- Kilocalories to kilojoules: multiply by 4.184. So 250 kcal is 1,046 kJ.
- Kilojoules to kilocalories: divide by 4.184, or multiply by 0.239. So 1,000 kJ is 239 kcal.
- Watt-hours to joules: multiply by 3,600. So 10 Wh is 36,000 J.
- Kilowatt-hours to megajoules: multiply by 3.6. So 2 kWh is 7.2 MJ.
- BTU to kilojoules: multiply by 1.055. So 10,000 BTU is 10,551 kJ.
- Kilowatt-hours to BTU: multiply by 3,412. So 3 kWh is 10,236 BTU.
A quick estimate for food: to change kilocalories into kilojoules, multiply by 4 and add 5%. For 500 kcal, 500 x 4 is 2,000, and 5% more is 2,100. The exact answer is 2,092 kJ.
Where these conversions are used
- Food and diet: labels in some countries show kcal and in others kJ.
- Electricity: bills are in kWh, and small devices are rated in Wh.
- Heating and cooling: boilers, heaters and air conditioners are often rated in BTU.
- Batteries: a battery's capacity is often given in Wh or kWh.
- School and science: joules are the standard unit of energy.
Tips
- The calorie on a food label is a kilocalorie. A food "Calorie" with a capital C is 1,000 of the small calories (cal). This converter keeps them apart: "calories" are the small ones.
- kW and kWh are different. A kilowatt (kW) is power, which is how fast energy is used. A kilowatt-hour (kWh) is energy, which is power used over time.
- BTU and BTU per hour are different. A BTU is an amount of energy. An air conditioner rated in BTU/h is a rating of power.
- Check the digits. A joule is very small, so an everyday amount such as a kilowatt-hour is millions of joules.
What this converter does not include
- Power units, such as watts, kilowatts and horsepower.
- Therms, electronvolts, foot-pounds and other special units.
- Other kinds of BTU. This converter uses 1 BTU = 1,055.06 joules.
This tool gives an estimate for general information only. The energy in real food or fuel can differ from the label or the typical value. For medical, safety or legal needs, follow the official guidance.