Energy Converter: Joules, Kilowatt-hours, Calories, and BTU
An energy converter expresses an energy quantity in a different unit. Convert between joules and their multiples, watt-hours, kilowatt-hours, megawatt-hours, thermochemical calories, kilocalories, and International Table BTU.
Changing units changes the numerical representation of energy rather than the energy quantity itself. Select the source and target units, enter a value, and compare the equivalent results.
How energy unit conversion works
Each supported unit has a fixed factor in joules. Multiply the input by its source factor, then divide by the target factor.
Converted energy = input energy × source joule factor ÷ target joule factor
The joule is the SI unit of energy. One kilojoule is 1,000 J, one megajoule is 1,000,000 J, and one gigajoule is 1,000,000,000 J. Energy conversion factors are listed in NIST’s conversion-factor reference.
Converting watt-hours to joules
A watt is one joule per second. Since an hour contains 3,600 seconds, one watt-hour equals 3,600 joules.
1 Wh = 3,600 J
1 kWh = 3,600,000 J = 3,600 kJ = 3.6 MJ
1 MWh = 1,000 kWh = 3.6 GJ
Power and energy are different quantities. A kilowatt measures a rate of energy use; a kilowatt-hour measures energy used over time. A device drawing 2 kW continuously for three hours uses 6 kWh.
Calories versus kilocalories
This page uses the thermochemical calorie: 1 cal = 4.184 J. One kilocalorie is 1,000 calories, so 1 kcal = 4,184 J = 4.184 kJ. The thermochemical definition is identified in NIST’s unit-definition notes.
The capital-C Calorie used for food energy corresponds to a kilocalorie. Choose kcal when converting that quantity. For example, 100 kcal converts to 418.4 kJ, while 100 small calories converts to 418.4 J.
There are other historical calorie definitions. These options consistently use thermochemical calories rather than silently mixing definitions.
Which BTU definition is used?
The BTU option uses the International Table definition, with 1 BTU IT = 1,055.05585262 J. This factor is given in NIST’s conversion notes.
BTU and BTU per hour describe different quantities: BTU measures energy, while BTU/h measures power. This page converts energy values. It does not convert a power rating directly to an energy amount without a time interval.
Worked energy conversion examples
2.5 kWh to megajoules: 2.5 × 3.6 = 9 MJ. 500 kJ to kilocalories: 500 ÷ 4.184 is approximately 119.5028680688 kcal.
1,000 BTU IT to kilojoules: 1,000 × 1,055.05585262 ÷ 1,000 = 1,055.05585262 kJ.
7,200 J to watt-hours: 7,200 ÷ 3,600 = 2 Wh. 0.25 MWh to kilowatt-hours: 0.25 × 1,000 = 250 kWh.
Frequently asked questions
Can I convert energy directly to power?
No. Energy-to-power calculations require a time interval. Divide energy in joules by time in seconds to obtain average power in watts.
Can I enter a negative energy value?
Yes. The sign is preserved during conversion. Negative values can represent signed changes in energy in an appropriate physical context.
Does this calculate an electricity bill?
No. It converts energy quantities. A cost calculation also needs a price per unit of energy.
What happens when the source and target units are the same?
The numerical value stays unchanged.
Why are some results rounded or shown in scientific notation?
Browser arithmetic has finite precision. Displayed values use up to ten decimal places; very small and very large numbers use scientific notation.
What does Swap do?
After a calculation, it uses the unrounded result as the input and reverses the units. Before a calculation, it reverses the units while keeping the input value.