Energy converter guide
The joule (J) is the SI unit of energy. This converter uses joules as a common base: the source amount is multiplied by its joules-per-unit factor and the result is divided by the target factor.
Heat-unit names can hide different conventions. The choices here explicitly use the thermochemical calorie and kilocalorie and the International Table British thermal unit. Watt-hours use 1 Wh = 3600 J, while an electronvolt uses the exact elementary-charge value in the current SI.
How to convert energy units
- Enter energy: Enter a finite numeric amount; a negative sign is preserved mathematically.
- Choose the source convention: Select the full unit name, especially for calories and British thermal units.
- Choose the target: Select the destination unit or use Swap to reverse the pair.
- Convert: Select Convert energy to calculate through the joule base factor.
Formula and variables
Converting through joules gives one traceable path for every supported unit pair.
target energy = source energy × (joules per source unit) ÷ (joules per target unit)- Eₛ — Source energy
- The signed finite value entered in the source unit.
- fₛ — Source factor
- Joules represented by one source unit.
- fₜ — Target factor
- Joules represented by one target unit.
Convert kilowatt-hours to megajoules
Convert 1 kWh of electrical energy to MJ.
- Amount
- 1
- From
- kWh
- To
- MJ
- 1 kWh = 1000 W × 3600 s = 3,600,000 J
- 3,600,000 J ÷ 1,000,000 J/MJ
Result: 1 kWh = 3.6 MJ.
The two values describe exactly the same amount of energy in different units.
Understanding your results
Unit identity matters
A result is reproducible only when calₜₕ, Btu_IT, and horsepower-related mechanical units retain their stated conventions.
Signed values
The converter preserves negative values, which can represent direction of transfer or a balance convention. Whether that sign is physically meaningful depends on the application.
Assumptions
- The thermochemical calorie equals exactly 4.184 J.
- The International Table British thermal unit equals 1055.05585262 J.
- The electronvolt factor uses an elementary charge of exactly 1.602176634 × 10^-19 C.
- Floating-point arithmetic is used after the listed factors are applied.
Limitations
- The calculator converts quantities, not energy prices, efficiencies, or power over a changing time profile.
- It does not include every historical calorie or BTU convention.
- Displayed digits do not increase the precision of a measured input.
- Extreme magnitudes can exceed the JavaScript numeric range.
Common mistakes
- Confusing energy in kWh with power in kW.
- Treating food kilocalories and small calories as the same numeric unit.
- Using an unspecified BTU convention.
- Dropping the time multiplier when converting Wh to J.
- Reporting a result without its unit symbol.
Practical use cases
Electrical energy
Compare watt-hours, kilowatt-hours, megawatt-hours, and joules for batteries, meters, or generation totals.
Thermal and mechanical work
Translate explicitly defined calories, BTU, therms, and foot-pound force into SI energy.
Planning and decision guide
Preserve the source convention
Copy the complete unit label from the source document before converting. If a calorie or BTU convention is not stated, resolve that ambiguity instead of assuming the labels are interchangeable.
Frequently asked questions
How many joules are in 1 kWh?
Exactly 3,600,000 J because 1 kW is 1000 J/s and 1 hour is 3600 seconds.
Is a food Calorie the same as a calorie?
A food Calorie commonly means one kilocalorie, or 1000 small calories. This converter labels the thermochemical units explicitly.
Which BTU does this calculator use?
It uses the International Table British thermal unit, labeled Btu_IT.
Can energy be negative?
The arithmetic accepts a signed value. A negative energy change can be meaningful under a chosen transfer or balance convention.
Are kW and kWh interchangeable?
No. kW measures power, while kWh measures energy accumulated over time.
Sources and review
- NIST Guide to the SI, Appendix B.9: Factors for Units Listed by Kind of Quantity or Field of Science — National Institute of Standards and Technology. Accessed 2026-08-29.
- CODATA Value: elementary charge — National Institute of Standards and Technology. Accessed 2026-08-29.
Reviewed 2026-08-29.