Energy Converter - Joules, kWh, Calories & BTU

About the Energy Converter

Energy appears on electricity bills in kilowatt-hours, on food labels in kilojoules and kilocalories, on boilers and air conditioners in BTU, on gas bills in therms or kilowatt-hours, and in physics in joules and electronvolts. They all measure the same thing — the capacity to do work or produce heat — so converting between them lets you compare a meal with a light bulb, a gas bill with an electricity bill, or a heater's rating with its running cost.

This energy converter converts between eleven energy units: joules, kilojoules, megajoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU, therms, foot-pounds and electronvolts. It shows the result, the exact factor both ways, the energy in every other unit, and how long a 1,000 watt heater would take to use the same amount.

How to Use the Energy Converter

Enter the amount of energy.

Choose the unit to convert from and to.

The result appears with the factor, all the other units and the heater comparison.

How Energy Conversion Works

Every unit is converted through the joule:

  1 kJ    = 1,000 J               1 MJ   = 1,000,000 J
  1 cal   = 4.184 J               1 kcal = 4,184 J   (a food Calorie)
  1 Wh    = 3,600 J               1 kWh  = 3,600,000 J
  1 BTU   = 1,055.056 J           1 therm = 100,000 BTU = 105.506 MJ
  1 ft·lbf = 1.35582 J            1 eV   = 1.602177 × 10⁻¹⁹ J

  result = value × (from-unit in J) ÷ (to-unit in J)

A watt is one joule per second, so a kilowatt-hour — a thousand watts for 3,600 seconds — is 3.6 million joules.

Step-by-Step Example

1 kWh to BTU.

  To joules:   1 × 3,600,000 = 3,600,000 J
  To BTU:      3,600,000 ÷ 1,055.056 = 3,412.14 BTU

A kilowatt-hour of electricity is about 3,412.14 BTU, or 860.421 kcal.

2,000 food Calories to megajoules.

  2,000 kcal × 4,184 = 8,368,000 J = 8.368 MJ

A typical day's food, 2,000 kcal, is 8.368 MJ or about 2.32444 kWh — the same energy a 1,000 watt heater uses in about 2 hours 19 minutes.

Quick Reference

  1 kWh   = 3.6 MJ = 3,412.14 BTU = 860.421 kcal
  1 kcal  = 4.184 kJ = 1.16222 Wh
  1 MJ    = 0.277778 kWh = 239.006 kcal
  1 BTU   = 1.05506 kJ = 0.252164 kcal
  1 therm = 29.3071 kWh

Food Energy

Food labels show energy per serving or per 100 grams. In the EU, UK and Australia they give both kilojoules and kilocalories; in the US, just "Calories", which are kilocalories. The conversion is 4.184 kJ per kcal, so a 250 kcal snack is about 1,046 kJ. The body uses this energy far less efficiently than a heater: of the food energy used during exercise, only around a quarter ends up as mechanical work, and the rest becomes heat.

Household Energy

Electricity is billed by the kilowatt-hour, and gas in many countries now too, after converting the volume of gas burned using its calorific value. In the US, gas may be billed in therms, each 29.3071 kWh. Converting both to kWh lets you compare costs directly: if electricity costs four times as much per kWh as gas, an electric heater costs roughly four times as much to run as a gas one of the same output, though a heat pump can deliver three or more kWh of heat for each kWh of electricity it uses.

Heating and Cooling Ratings

Boilers, furnaces and air conditioners are often rated in BTU per hour, a unit of power rather than energy. A 12,000 BTU per hour air conditioner removes 12,000 BTU of heat each hour it runs, which is about 3.5 kWh of heat an hour. To compare with electrical ratings in watts, use a power converter, since power is energy per unit of time.

Tiny and Enormous Energies

Physicists use the electronvolt for the energy of single particles: the energy an electron gains crossing one volt, 1.602 × 10⁻¹⁹ joules. Visible light photons carry about 2 to 3 eV each. At the other extreme, national energy statistics use terawatt- hours and exajoules, where a terawatt-hour is a billion kilowatt-hours.

Energy in Fuels

Fuels are often compared by the energy they release when burned. A litre of petrol holds about 32 to 34 megajoules, roughly 9 kWh, and a litre of diesel slightly more. Natural gas gives about 38 megajoules per cubic metre, around 10.5 kWh, depending on its composition. A kilogram of dry firewood releases around 15 to 19 megajoules. These are typical values rather than exact definitions, but converting them to a common unit such as the kilowatt-hour shows how much energy different fuels deliver per litre, cubic metre or kilogram, and makes it much easier to compare running costs between heating systems or vehicles.

Understanding Your Result

The headline is the converted energy.

The conversion factor shows the exact factor both ways.

The in every unit line gives the energy in all eleven units.

The also line shows the energy in kWh, kcal and BTU, and how long a 1,000 W heater would take to use it.

When Should You Use This Calculator?

Use it to compare food energy on labels in kJ and kcal.

Use it to compare gas and electricity bills and heating fuels.

Use it with heating, cooling and water-heating ratings in BTU.

Use it for physics and chemistry calculations in joules and electronvolts.

Common Mistakes

Confusing calories and Calories. A food Calorie is a kilocalorie, 1,000 calories.

Mixing up kW and kWh. Kilowatts are power; kilowatt-hours are energy.

Treating BTU and BTU per hour as the same. One is energy, the other power.

Using a different calorie. This converter uses the thermochemical calorie, 4.184 J; the International Table calorie is 4.1868 J, about 0.07 percent larger.

Rounding factors too early. Use 3.6 MJ per kWh and 4.184 kJ per kcal exactly, and round only the final answer.

Forgetting efficiency. Converting units does not account for energy lost in boilers, engines or bodies.