Energy Converter

Food labels use kilocalories or kilojoules, electricity bills use kilowatt-hours, heating specs use BTU, and physics uses joules. This converter handles all of them plus calories, watt-hours, foot-pounds and electronvolts, so comparing a snack with a battery or a heating bill stops being a research project.

How to use it

1. Enter the amount of energy.
2. Pick the unit you have and the one you want.
3. Read the headline result and the table below it.
4. Swap units to reverse.

When it helps

• Switching food energy between kilocalories and kilojoules.
• Making sense of electricity use in kilowatt-hours.
• Comparing heating and cooling ratings quoted in BTU.
• Physics and chemistry homework.

Good to know

• The Calorie on a nutrition label is a kilocalorie, which is 1,000 of the small calories scientists use.
• One kilocalorie is 4.184 kilojoules, so 250 kcal is roughly 1,046 kJ.
• A kilowatt-hour is 3.6 million joules, which is a useful way to feel how big a joule is not.

How the conversion works

The joule is the base unit. A calorie is defined as 4.184 joules, and a kilocalorie as 4,184. A watt-hour is 3,600 joules, since a watt is one joule per second and there are 3,600 seconds in an hour. A kilowatt-hour is therefore 3.6 million joules. A BTU is about 1,055 joules, and a foot-pound about 1.356.

The electronvolt is the energy gained by an electron crossing one volt, about 1.6 times ten to the minus nineteen joules. It's used for atoms and particles, where joules would be inconveniently huge.

Worked example: a snack versus a battery

A 250 kcal snack equals 1,046 kJ, or 0.29 kWh. A phone battery holds about 15 Wh, so one snack contains roughly twenty times the energy of a phone battery. Of course your body doesn't use it as efficiently or as neatly as a charger does, but it puts the numbers in perspective.

On an electricity bill, 1 kWh may cost around 0.15 in many countries. A 2,000-watt heater running for one hour uses 2 kWh, or about 7.2 million joules, which is also 1,720 kcal.

Calories confusion

The word "calorie" is used in two ways. In physics a calorie heats one gram of water by one degree Celsius. In food, "Calorie" with a capital C, or "kcal", means a thousand of those. A packet that says 250 Calories means 250 kilocalories.

Outside the US and UK, many labels give kilojoules as well or instead. Multiply kilocalories by 4.184 to get kilojoules, or divide kilojoules by 4.184 to go back.

Heating and energy bills

Heating systems are often rated in BTU per hour for power, but gas is billed in energy units such as therms or kilowatt-hours. One therm is 100,000 BTU, about 29.3 kWh. To compare fuel costs, convert everything to kilowatt-hours and divide the price by the energy.

Efficiency changes the picture: a gas boiler at 90 percent efficiency delivers 0.9 kWh of heat per kWh of gas, while a heat pump may deliver three or more. The converter gives you the raw numbers, and efficiency is the next step.

Questions