Energy Unit Conversion: Joules, kWh and More
Energy is one of the most important quantities in science, engineering and everyday life. Yet the units used to measure it vary enormously depending on the context. Your electricity bill uses kilowatt-hours. Your food label uses kilocalories. Your gas bill might use therms. A physics textbook uses joules. A heating engineer might use BTU.
This guide explains every major energy unit, shows you how to convert between them and links to the free Energy Converter on CalConvs.
The Major Energy Units
| Joule (J) | The SI unit of energy. Used in physics and science. |
| Kilojoule (kJ) | 1,000 joules. Common in chemistry and nutrition. |
| Kilowatt-hour (kWh) | Energy equivalent to one kilowatt operating for one hour. Used for electricity billing worldwide. |
| Kilocalorie (kcal) | 1,000 calories. The food calorie shown on nutrition labels. Often written as Calorie with a capital C. |
| BTU (British Thermal Unit) | Energy to raise 1 pound of water by 1 degree Fahrenheit. Used in HVAC and gas appliances in the US and UK. |
| Therm | 100,000 BTU. Used in gas billing in the US. |
| Electronvolt (eV) | Extremely small unit used in atomic and particle physics. |
Key Conversion Factors
Core Energy Conversions
1 kWh = 3,600,000 J = 3,600 kJ = 3,412.14 BTU = 859.845 kcal
1 BTU = 1,055.06 J = 1.055 kJ = 0.000293 kWh = 0.252 kcal
1 kcal = 4,184 J = 4.184 kJ = 3.966 BTU = 0.001163 kWh
1 kJ = 1,000 J = 0.239 kcal = 0.948 BTU = 0.000278 kWh
Energy Conversion Reference Table
| From | Joules | Kilocalories |
|---|---|---|
| 1 J | 1 J | 0.000239 kcal |
| 1 kJ | 1,000 J | 0.239 kcal |
| 1 kcal | 4,184 J | 4.184 kJ |
| 1 BTU | 1,055 J | 0.252 kcal |
| 1 kWh | 3,600,000 J | 860 kcal |
| 1 therm | 105,480,400 J | 25,200 kcal |
Food Energy: Calories on Nutrition Labels
The energy value on a nutrition label is always in kilocalories (kcal), even when it is labelled simply as calories or Cal with a capital C. One food Calorie is 1,000 chemistry calories and 4,184 joules.
| A small apple (150g) | About 78 kcal / 326 kJ |
| A slice of white bread | About 80 kcal / 335 kJ |
| A tablespoon of olive oil | About 120 kcal / 502 kJ |
| A chicken breast (150g grilled) | About 230 kcal / 963 kJ |
| A can of regular cola (330ml) | About 139 kcal / 582 kJ |
| A pint of lager (568ml) | About 182 kcal / 762 kJ |
If you are tracking daily calorie intake, the Calorie Calculator and Macro Calculator give you personalised daily targets.
Electricity: Understanding kWh on Your Bill
Your electricity meter measures energy in kilowatt-hours (kWh). One kWh is the energy consumed by a 1,000-watt device running for one hour, or a 100-watt device running for 10 hours.
Electricity Cost Calculation
A 100-watt light bulb running for 10 hours uses 1 kWh.
A 2,000-watt kettle running for 30 minutes uses 1 kWh.
Formula: Cost = Power in kW multiplied by Hours multiplied by Price per kWh
Example: Running a 3 kW electric shower for 8 minutes at 30 cents per kWh:
3 kW multiplied by (8 divided by 60) hours multiplied by 0.30 = $0.12 per shower
BTU in Heating and Cooling
BTU is the standard unit for heating and cooling capacity in the US. Air conditioners, furnaces and boilers are rated in BTU per hour.
| 5,000 BTU/hr | Small window air conditioner for a room up to about 150 square feet |
| 8,000 BTU/hr | Window AC for a room up to about 350 square feet |
| 12,000 BTU/hr | 1 ton of cooling capacity. Standard measure in HVAC. |
| 24,000 BTU/hr | 2 ton unit. Suitable for a medium-sized home. |
| 36,000 BTU/hr | 3 ton unit. Suitable for a larger home. |
Related Tools
- Energy Converter joules, kWh, BTU, kcal, kJ and more
- Calorie Calculator daily energy needs for health and fitness
- All Unit Converters browse all 74 converters on CalConvs
Frequently Asked Questions
How many joules are in a kilowatt-hour?
One kilowatt-hour equals 3,600,000 joules (3.6 megajoules). This comes from the definition: 1 kW is 1,000 joules per second, and 1 hour is 3,600 seconds. So 1,000 joules per second multiplied by 3,600 seconds equals 3,600,000 joules. This is why electricity meters use kWh rather than joules, the numbers would be impractically large in joules.
What is the difference between a calorie and a kilocalorie?
A calorie (lowercase c) is the energy needed to raise 1 gram of water by 1 degree Celsius. A kilocalorie (kcal) is 1,000 of these small calories. Food labels use kilocalories, but they often label them as Calories (uppercase C) or simply calories, which causes widespread confusion. When a nutrition label says a food has 200 calories, it actually means 200 kilocalories, or 200,000 small calories.
How do I calculate my electricity cost from kWh?
Multiply the power rating of your device in kilowatts by the hours of use, then multiply by your electricity price per kWh. For example: a 2 kW device running for 3 hours at 25 cents per kWh costs 2 multiplied by 3 multiplied by 0.25 = $1.50. Check your electricity bill for your rate per kWh, which varies significantly by country and supplier.
What does BTU stand for and when is it used?
BTU stands for British Thermal Unit. It is defined as the energy needed to raise the temperature of one pound of water by one degree Fahrenheit. Despite the name, BTU is used primarily in the United States today, particularly for rating heating and cooling equipment such as air conditioners, furnaces, boilers and water heaters. 12,000 BTU per hour equals one ton of cooling, a common HVAC industry measure.