Energy Converter

About this Energy Converter

The Energy Converter translates energy quantities between common scientific, electrical and food-energy units. It can help compare specifications and calculations that use different measurement systems.

Energy is measured very differently across contexts: physics and engineering typically use joules, nutrition labels use calories, and electricity bills use kilowatt-hours, so converting between them is necessary whenever you compare energy figures from different fields.

For related tasks, try Calorie Calculator, Power Converter or Pressure Converter.

How the calculation works

Conversions use joules as the internal base unit. Food kilocalories are represented as kcal, while small calories use cal.

How to use it

1. Enter an energy value.
2. Select the original and target units.
3. Convert to see the equivalent amount.

Practical example

1 kilowatt-hour equals 3,600,000 joules.

A food item listing 200 Calories on its nutrition label equals about 836,800 joules (since 1 food Calorie is approximately 4,184 joules), a number so large it illustrates why joules are rarely used directly on food packaging.

Things to double-check

A food label's Calorie (capital C) is actually a kilocalorie (1,000 small calories), a naming convention that frequently causes confusion when converting nutrition energy figures into scientific joules or comparing them against other calorie-based figures.

Joules, calories, kilowatt-hours and BTU

One kilowatt-hour is 3.6 million joules or 3,600 kilojoules, about 860 kilocalories. One kilocalorie, the unit on food labels, is 4.184 kilojoules, so a 2,000 kcal daily diet is about 8,368 kJ. One BTU is about 1,055 joules. The joule is the standard scientific unit, the kilowatt-hour appears on electricity bills, calories are used for food and BTU is used for heating and cooling equipment. This converter links them.

Reading electricity use

Energy used equals power times time. A 1,500 watt heater running for 3 hours uses 4.5 kilowatt-hours. At a price of 0.15 per kilowatt-hour that costs about 0.68. To estimate monthly use, multiply the appliance's power in kilowatts by the hours it runs and then by the days. Converting between kilowatt-hours and other units helps you compare, for example, how much food energy equals an hour of a heater.

Food labels

Nutrition labels may show energy in kilojoules, kilocalories or both. A snack labelled 840 kJ is 200 kcal, because 840 divided by 4.184 is about 200. People usually mean kilocalories when they say calories in everyday speech. For diet planning, compare the label with your estimated daily energy needs, which vary with age, size and activity, and use a professional's advice for medical needs.

Heating and cooling equipment

Air conditioners and heaters are rated in BTU per hour, and boilers and furnaces may be rated in kilowatts. A 12,000 BTU per hour air conditioner removes heat at about 3.5 kilowatts. When you compare equipment from different markets, convert the ratings to the same unit and also look at efficiency, which tells you how much electricity is needed for that output. A higher rating is not better if it is oversized for your room.

Fuel and electricity compared

Different energy sources can be compared in the same unit. A litre of petrol contains about 34 megajoules, which is about 9.4 kilowatt-hours. An electric car typically uses 15 to 20 kilowatt-hours per 100 km, while a petrol car using 7 litres per 100 km consumes about 66 kilowatt-hours of fuel energy for the same distance, though much of that is lost as heat. Converting to a common unit makes the comparison clearer, but remember to include conversion losses and the cost per unit for a fair comparison.

Battery capacity

Battery sizes are an everyday use of energy units. A phone battery of 5,000 mAh at 3.85 volts stores about 19.25 watt-hours, which is 0.019 kilowatt-hours. A laptop battery of 60 watt-hours holds more than three times as much. Airlines commonly limit spare lithium batteries to 100 watt-hours each in carry-on luggage, so converting a capacity from milliamp-hours to watt-hours lets you check the rule. An electric car with a 60 kilowatt-hour battery stores about 3,000 times the energy of the phone.

Worked example: estimating a monthly energy cost

An air conditioner rated at 1.5 kilowatts that runs 6 hours a day for 30 days uses 1.5 times 6 times 30, which is 270 kilowatt-hours. At a price of 0.14 per kilowatt-hour, that costs 37.80. A refrigerator rated at 150 watts that runs about 40 percent of the time uses 0.15 times 24 times 30 times 0.4, or about 43 kilowatt-hours, costing about 6. Adding such estimates for your main appliances shows where the money goes. You can then convert the total to megajoules or kilocalories if you want to compare it with other energy sources or with food energy.

Food energy and exercise

Food labels give energy in kilojoules, kilocalories or both. A 100 gram bar of chocolate with 540 kilocalories contains about 2,260 kilojoules. A 70 kilogram person walking briskly for 30 minutes uses roughly 120 kilocalories, using the common estimate of 3.5 metabolic equivalents. That means a 540 kilocalorie bar would take over two hours of brisk walking to burn off. These are averages that vary with body size, age and fitness, so treat them as rough guides, and use them to understand scale rather than to count every kilocalorie.

Energy content of fuels

Fuels differ in how much energy they hold. Petrol contains about 34 megajoules per litre, diesel about 36 and natural gas about 38 megajoules per cubic metre, which is roughly 10.5 kilowatt-hours. Liquefied petroleum gas has about 46 megajoules per kilogram. Wood and coal vary widely with moisture and quality. Converting fuels to the same unit lets you compare costs per kilowatt-hour for heating or cooking. Remember to include the efficiency of the appliance: a boiler that is 90 percent efficient delivers 90 percent of the fuel energy as heat.

Units used in the energy industry

Energy is quoted in many units. Electricity bills use kilowatt-hours, gas bills may use cubic metres, therms or kilowatt-hours, and heating equipment uses British thermal units. A therm is 100,000 BTU, about 105.5 megajoules. Large energy figures may be given in gigajoules, terajoules or barrels of oil equivalent, where a barrel is about 6.1 gigajoules. A bill that shows cubic metres needs a conversion factor from the supplier to turn it into kilowatt-hours. Understanding these units helps you read bills and compare suppliers fairly.

Key terms explained

A joule is the standard unit of energy, and a kilojoule is a thousand joules. A calorie is about 4.184 joules, and the kilocalorie used on food labels is a thousand calories. A watt-hour is the energy of one watt used for an hour, and a kilowatt-hour is 3.6 million joules. A British thermal unit is about 1,055 joules. Power is energy per unit of time, and efficiency is the share of input energy that becomes useful output.

Rules of thumb and quick checks

One kilowatt-hour equals 3.6 megajoules or about 860 kilocalories. One kilocalorie is 4.184 kilojoules, so divide a kilojoule figure by 4.2 to estimate kilocalories. A 100 watt device uses 0.1 kilowatt-hours in an hour. To estimate electricity cost, multiply power in kilowatts by hours of use by the price per kilowatt-hour. Remember efficiency: a heater turns almost all electricity into heat, while a petrol engine turns most fuel energy into waste heat.

Quick reference and sanity checks

Several anchors catch most mistakes. One joule is one watt for one second, one kilowatt hour equals exactly 3.6 million joules and one kilocalorie equals 4,184 joules. The food Calorie with a capital C on a label is a kilocalorie. A British thermal unit is about 1,055 joules. Energy is not the same as power, so a kilowatt hour measures how much was used and a kilowatt measures how fast. Utility bills charge for kilowatt hours, while appliance labels show watts. If a result looks a thousand times too large or too small, check whether a prefix such as kilo or mega was lost.

Frequently asked questions