7 units · 42 conversions

Energy Converter

Convert joules, calories, kilowatt-hours and BTU.

Popular Energy conversions

Joules to Kilojoules J → kJ Joules to Calories J → cal Joules to Kilocalories J → kcal Joules to Watt-hours J → Wh Joules to Kilowatt-hours J → kWh Joules to BTU J → BTU Kilojoules to Joules kJ → J Kilojoules to Calories kJ → cal Kilojoules to Kilocalories kJ → kcal Kilojoules to Watt-hours kJ → Wh Kilojoules to Kilowatt-hours kJ → kWh Kilojoules to BTU kJ → BTU Calories to Joules cal → J Calories to Kilojoules cal → kJ Calories to Kilocalories cal → kcal Calories to Watt-hours cal → Wh Calories to Kilowatt-hours cal → kWh Calories to BTU cal → BTU Kilocalories to Joules kcal → J Kilocalories to Kilojoules kcal → kJ Kilocalories to Calories kcal → cal Kilocalories to Watt-hours kcal → Wh Kilocalories to Kilowatt-hours kcal → kWh Kilocalories to BTU kcal → BTU Watt-hours to Joules Wh → J Watt-hours to Kilojoules Wh → kJ Watt-hours to Calories Wh → cal Watt-hours to Kilocalories Wh → kcal Watt-hours to Kilowatt-hours Wh → kWh Watt-hours to BTU Wh → BTU

All Energy conversions (42)

All Energy units

Joules J Kilojoules kJ Calories cal Kilocalories kcal Watt-hours Wh Kilowatt-hours kWh BTU BTU

About Energy

Energy covers seven units and they come from three different worlds that rarely talk to each other. Joules and kilojoules are the SI units used in physics and on food labels in much of the world. Calories and kilocalories are the nutrition units, and the ones on American and British food packaging. Watt-hours and kilowatt-hours are the electricity units: your power bill is denominated in kWh and so is every battery spec. BTU is the heating and cooling unit, which is why air conditioners and boilers are rated in it. People land here to compare a food label written in the wrong unit, work out what an appliance costs to run, size a heater, or answer a physics question. The pages used most are kilojoules to kilocalories, kilocalories to kilojoules, kilowatt-hours to joules and joules to calories.

The single biggest source of confusion here is the word calorie. In physics, a calorie is a small unit of heat. In nutrition, the Calorie people talk about on a food label is actually a kilocalorie, a thousand of the physics ones. So when a label says a meal contains a few hundred calories, it means kilocalories, and a physicist reading the same number literally would be out by a factor of a thousand. This site keeps calories and kilocalories as separate units precisely so you can be explicit about which one you mean. The other thing worth knowing is that energy is not power. Energy is a total amount; power is the rate at which it is used. A kilowatt-hour is an energy unit built from a power unit and a time, which is why a device rated at some number of watts consumes kilowatt-hours over the course of an hour rather than being measured in them. If you are comparing how hard a device works rather than how much it consumes overall, the power converter is the hub you need.

Each pair page shows the formula it applied, a worked example, and a reference table of common values. Everything is free, no sign-up is required, and every calculation runs entirely in your browser, so nothing you type is sent to a server.

Frequently asked questions

Is a food calorie the same as a calorie in physics?

No, and this is the most common mix-up in the whole category. The Calorie on a food label is really a kilocalorie, a thousand of the small calories used in physics. So a snack listed at a few hundred calories contains that many kilocalories. This site lists calories and kilocalories as separate units so you can be explicit about which one your source means and avoid being wrong by a factor of a thousand.

What is the difference between energy and power?

Energy is a total quantity, while power is the rate at which energy is used. A joule or a kilowatt-hour measures how much; a watt measures how fast. A kilowatt-hour is what you get when a device drawing one kilowatt runs for one hour, which is why bills are in kilowatt-hours but appliance ratings are in watts. For rate-based conversions, use the power converter instead.

Why are appliances rated in BTU?

BTU is the traditional heating and cooling unit, and the heating and air conditioning industry has kept it, particularly in the US and UK. So air conditioners, boilers and furnaces are commonly rated in BTU or BTU per hour rather than in watts or kilowatts. Converting to joules or kilowatt-hours makes it easier to compare an imported spec against a metric one, which is what the BTU pages are for.

Are food labels in kilojoules or calories?

It depends on the country. Australia, New Zealand and much of Europe put kilojoules on packaging, sometimes alongside kilocalories, while US and UK labels lead with calories, meaning kilocalories. That is why the kilojoules to kilocalories page and its reverse get so much use: people are comparing a product bought in one place against a figure they are used to reading from another.

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