Energy Converter

Energy is the capacity to do work. The joule is the SI unit, but energy is also measured in calories, kilowatt-hours, British thermal units (BTU) and electronvolts depending on the field.

This converter is useful for physics, nutrition, utility bills and heating, where the same quantity of energy is expressed in very different units.

Two units share the name calorie and they differ by a factor of a thousand. The scientific calorie is 4.184 joules: the energy to raise one gram of water by one degree Celsius. The food Calorie, sometimes capitalised to signal the difference, is the kilocalorie: 4,184 joules.

Every nutritional figure in everyday use is the larger one. A snack listed as 250 calories contains 250 kilocalories, or about 1,046,000 joules, roughly 0.29 kilowatt-hours. Reading it as scientific calories understates the energy content by a factor of 1,000.

• Labels in most of the world print kilojoules alongside or instead, and no ambiguity exists there.
• Exercise equipment showing "calories burned" also means kilocalories.
• This converter lists calorie and kilocalorie separately. Selecting the wrong one is the most common error on this page.

A kilowatt is a rate; a kilowatt-hour is an amount. Power describes how fast energy is being used, energy describes how much has been used, and a bill charges for the second. Running a 2 kW heater for three hours consumes 6 kWh regardless of how quickly it heated the room.

Everything on this page is energy. Watts, kilowatts and horsepower belong on the power converter, and the two cannot be converted into one another without a time period to multiply or divide by.

• One kilowatt-hour is 3,600,000 joules, about 860 kilocalories, or roughly 3,412 BTU.
• Gas is frequently billed by volume and converted to kWh by the supplier using a calorific value, which is why the same gas volume can bill differently.
• The BTU is still standard in US heating and air conditioning, where equipment is rated in BTU per hour, again a rate, not a quantity.