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ToolBoxGenie

Energy Converter

Converters · Added 15 August 2026

Convert energy between fifteen units spanning physics, electricity bills, heating systems and nutrition labels. The one to watch is the calorie: the Calorie on a food packet is a kilocalorie, a thousand times the calorie a physicist means, and both are listed here separately.

Result

1 kWh in megajoule

3.6 MJ

1 kWh = 3.6 MJ

The same value in every unit

Joule (J)
3,600,000
Kilojoule (kJ)
3,600
Megajoule (MJ)
3.6
Gigajoule (GJ)
0.0036
Watt-hour (Wh)
1,000
Kilowatt-hour (kWh)
1
Megawatt-hour (MWh)
0.001
Calorie (thermochemical) (cal)
860,420.65
Kilocalorie / food Calorie (kcal)
860.42065
British thermal unit (BTU)
3,412.14163
Therm (US) (thm)
0.0341295634
Foot-pound (ft·lbf)
2,655,223.74
Erg (erg)
36,000,000,000,000
Electronvolt (eV)
2.2469e+25
Tonne of oil equivalent (toe)
0.0000859845228

The food Calorie on a nutrition label is a kilocalorie — a thousand of the calories a physicist means. Both are listed separately here, so pick kcal for anything to do with diet.

How to use the energy converter

  1. 1Enter the energy value you want to convert.
  2. 2Pick the unit it is in and the unit you want.
  3. 3For anything to do with food, use kilocalories rather than calories.
  4. 4The reference table shows the same quantity in every unit at once.
  5. 5Swap the units to reverse the direction.

Examples

An electricity bill in heating units

Input
1 kWh
Result
3.6 MJ · 3,412.14 BTU · 860.42 kcal

A kilowatt-hour is exactly 3.6 megajoules, because a watt is a joule per second and an hour is 3,600 seconds.

A day's food energy

Input
2,000 kcal
Result
8,368 kJ · 2.32 kWh

European labels give kilojoules, US and UK labels usually give kilocalories. The same meal, two scales.

Gas heating

Input
1 therm (US)
Result
105.48 MJ · 29.30 kWh

Gas is often billed in therms or in kWh depending on the country.

About the energy converter

One quantity, many historical units

Energy has more units in common use than almost any other quantity, and the reason is historical rather than technical. Heat, mechanical work, electricity and chemical energy were each studied separately before anyone established that they are the same thing, so each field invented its own measure. The BTU came from steam engineering, the calorie from the study of heat, the foot-pound from mechanics, the kilowatt-hour from electricity supply, and the electronvolt from atomic physics.

James Joule's experiments in the 1840s established the mechanical equivalent of heat and made all of these interconvertible. The joule is the SI unit that resulted, and it is defined cleanly as one newton acting over one metre. Everything else in this converter is a fixed multiple of it.

The units survive because each is conveniently sized for its field. Measuring a household electricity bill in joules would mean quoting figures in the hundreds of millions; measuring a particle collision in kilowatt-hours would require twenty leading zeros. The unit is chosen so the number lands in a range humans can read at a glance.

Reading an energy figure in context

Energy figures only mean something once you know what they are being compared against. A kilowatt-hour is roughly what a 10 W LED bulb uses in four days, what an electric kettle uses in about forty minutes, or what an electric car needs to travel around six kilometres. The same number sits in wildly different places depending on the context.

The most common error in practice is confusing an energy figure with a power figure — treating a 2 kW appliance as if it consumed 2 kWh regardless of how long it runs. Power tells you the rate; energy is what you are billed for, and it needs the duration as well. An appliance rating in watts becomes a cost only once multiplied by hours of use.

For heating, a second subtlety matters: the energy content of fuel is not the energy delivered to the room. A gas boiler at 90% efficiency turns 100 units of gas energy into 90 units of heat, and the rest goes up the flue. Converting between therms and kilowatt-hours converts the fuel, not the useful output, so comparing a gas figure with an electric one needs the efficiency of each system as well as the unit conversion.

Frequently asked questions

Is a food Calorie the same as a calorie?
No, and the difference is a factor of a thousand. The calorie used in physics is the energy needed to warm one gram of water by one degree Celsius — about 4.184 joules. The Calorie on a nutrition label, written with a capital C, is a kilocalorie: a thousand of those. A 500-Calorie meal contains 500,000 calories in the physics sense. Use the kilocalorie entry for anything dietary.
Which calorie definition does this use?
The thermochemical calorie, defined as exactly 4.184 joules. There is also an International Table calorie of 4.1868 J and a handful of others, differing in the fourth significant figure. The distinction matters in precision calorimetry and nowhere else; for food, heating and general work the thermochemical value is the standard choice.
What is the difference between energy and power?
Power is the rate at which energy is used — energy divided by time. A 2 kW heater consumes 2 kilowatt-hours of energy per hour of running. This is why a kilowatt-hour is a unit of energy despite containing the word 'watt': it is a rate multiplied by a duration. If you need to convert watts, horsepower or BTU per hour, use the power converter instead.
Why is the electronvolt so small?
Because it is the energy one electron gains crossing a one-volt potential difference, and an electron carries an extremely small charge — about 1.602 × 10⁻¹⁹ coulombs. That makes an electronvolt roughly 1.6 × 10⁻¹⁹ joules. It exists because particle physics deals in quantities where a joule would be an absurdly large unit, in the same way that you would not measure a hair's width in kilometres.

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