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Energy Converter

Convert energy between joules, kilojoules, megajoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU, electronvolts, therms and foot-pounds force.

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Convert an amount of energy

Read the guide Energy Units Explained: Joules, Calories, kWh and BTU

How to use the converter

Type an amount in the Value box, choose the unit you have and the unit you want, and press Convert. The swap button flips the two units, and a table below the result shows the same amount in all eleven units. The converter starts on kilowatt-hours to kilojoules.

Unit definitions

Every unit is converted through the joule (J), the SI unit of energy. The factors used are:

  • Kilojoule (kJ) = 1,000 J and megajoule (MJ) = 1,000,000 J.
  • Calorie (cal) = 4.184 J exactly. This is the thermochemical calorie, the one used in chemistry.
  • Kilocalorie (kcal) = 4,184 J. This is the "Calorie" with a capital C printed on food labels.
  • Watt-hour (Wh) = 3,600 J, the energy of 1 watt for one hour. Kilowatt-hour (kWh) = 3,600,000 J, the unit on electricity bills.
  • British thermal unit (BTU) = 1,055.05585262 J, the International Table definition.
  • Electronvolt (eV) = 1.602176634 × 10^-19 J, exact since the 2019 revision of the SI. It is used in atomic and particle physics.
  • Therm (US) = 105,480,400 J, used for natural gas billing in the United States. It equals 100,000 BTU when the BTU is measured at 59 °F; in International Table BTU it is about 99,976.13 BTU.
  • Foot-pound force (ft·lbf) = 1.3558179483314004 J, the work done by a force of one pound over one foot.

To convert, multiply by the factor of the starting unit and divide by the factor of the target unit.

Worked examples

  • 1 kWh to kJ: 3,600,000 J ÷ 1,000 = 3,600 kJ.
  • 1 kWh to BTU: 3,600,000 ÷ 1,055.05585262 ≈ 3,412.14 BTU, a figure often used for heating and air conditioning.
  • 2,000 kcal to kWh: 2,000 × 4,184 = 8,368,000 J, and 8,368,000 ÷ 3,600,000 ≈ 2.32 kWh. A typical day of food holds about as much energy as running a 1 kW heater for a little over two hours.
  • 1 therm to kWh: 105,480,400 ÷ 3,600,000 ≈ 29.30 kWh.

Good to know

  • Other calorie definitions exist, such as the International Table calorie (4.1868 J). They differ by less than 0.1%, which rarely matters outside a laboratory.
  • Watt-hours measure energy, while watts measure power. A 60 W bulb left on for 10 hours uses 600 Wh, or 0.6 kWh.
  • Results show up to six decimal places. Very small values are written in scientific notation; very large ones, such as kilowatt-hours expressed in electronvolts, are rounded to the precision a computer number can hold.
  • Energy amounts cannot be negative. Zero is accepted.

Frequently asked questions

How many kilojoules are in a kilowatt-hour?

Exactly 3,600 kJ. A kilowatt is 1,000 joules per second and an hour has 3,600 seconds, so 1 kWh = 3,600,000 J = 3,600 kJ.

Is a food Calorie the same as a calorie?

No. The Calorie on food labels is a kilocalorie, 1,000 small calories or 4,184 joules. Labels in many countries also list the energy in kilojoules: 100 kcal is 418.4 kJ.

How many BTU are in a kilowatt-hour?

About 3,412.14 BTU, using the International Table BTU of 1,055.05585262 joules. Heating and cooling equipment ratings are often converted with this figure.

What is a therm?

A therm is a unit used to bill natural gas. The US therm is defined as 105,480,400 joules, which is about 29.30 kWh or roughly 100,000 BTU.