Converters

Energy Converter

Convert any energy value between joules, kilojoules, megajoules, calories, kilocalories, watt-hours, kilowatt-hours, and BTU. All units displayed at once.

Enter a value and select a unit to convert all energy units at once.

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Energy Conversion Reference

All conversions use the joule (J) as the base unit. The table below shows the number of joules in each unit.

1 kJ = 1,000 J

1 MJ = 1,000,000 J

1 cal = 4.184 J

1 kcal = 4,184 J

1 Wh = 3,600 J

1 kWh = 3,600,000 J

1 BTU = 1,055.06 J

Energy Units in Context

Different fields have adopted different energy units for historical and practical reasons. Nutrition uses kilocalories because they produce round numbers for food portions. Electricity billing uses kilowatt-hours because they are convenient for the timescales and power levels of household appliances. Physics and chemistry use joules and kilojoules because they integrate seamlessly with SI units for force, mass, and time.

In the United States, BTUs dominate HVAC (heating, ventilation, and air conditioning) because the imperial unit system remains standard there. A 12,000 BTU air conditioner is rated to cool a 500 to 600 square foot room and equals approximately 3.5 kilowatts of cooling capacity. Fuel efficiency in some contexts is expressed as BTU per pound or per gallon.

At the global scale, national energy statistics are often reported in exajoules (EJ, 10¹⁸ joules) or petajoules (PJ, 10¹⁵ joules). World primary energy consumption in 2023 was approximately 600 EJ, equivalent to about 166,000 TWh (terawatt-hours) of electricity.

Frequently asked questions

What is the SI unit of energy?
The SI (International System of Units) unit of energy is the joule (J), named after physicist James Prescott Joule. One joule is the energy transferred when a force of one newton acts over a distance of one metre. The joule is also equal to one watt-second — the energy consumed by a one-watt device operating for one second. In practice, the joule is small for everyday energy contexts, so kilojoules (kJ), megajoules (MJ), and kilowatt-hours (kWh) are more commonly used.
What is the difference between a calorie and a kilocalorie?
The small calorie (cal) is the amount of energy needed to raise one gram of water by one degree Celsius at standard pressure. The kilocalorie (kcal), equal to 1000 small calories, is the unit used on food nutrition labels in most countries. Confusingly, nutrition labels in the United States often use the word 'Calorie' (with a capital C) to mean kilocalorie. So when a food label says 200 Calories, it means 200 kcal or 200,000 small calories. One kilocalorie equals approximately 4,184 joules.
How do you convert kilowatt-hours to joules?
One kilowatt-hour (kWh) is equal to 3,600,000 joules (3.6 MJ). This is because one watt equals one joule per second, one kilowatt is 1,000 watts, and one hour is 3,600 seconds. Therefore 1 kWh = 1,000 W × 3,600 s = 3,600,000 J. The kWh is the standard unit used by electricity providers to bill residential and commercial customers for electrical energy consumption.
What is a BTU and where is it used?
A British Thermal Unit (BTU) is a traditional unit of energy equal to approximately 1,055 joules. It is defined as the energy needed to raise one pound of water by one degree Fahrenheit. BTUs are widely used in the United States for rating the heating and cooling capacity of air conditioners, furnaces, and boilers. Natural gas and other fuel energy content is often expressed in BTU or therms (100,000 BTU). One BTU equals approximately 0.293 watt-hours.
How many joules are in a food Calorie?
One food Calorie (kilocalorie, kcal) equals exactly 4,184 joules or 4.184 kilojoules. A typical 2,000-kilocalorie daily diet therefore represents approximately 8.37 megajoules of energy. This is roughly equivalent to the energy in 2.3 kWh of electricity. The human body converts this chemical energy from food into mechanical work, heat, and metabolic processes with an efficiency of roughly 25 percent for muscular work.
What is the difference between energy and power?
Energy is the total amount of work done or transferred, measured in joules or kilowatt-hours. Power is the rate at which energy is transferred or used, measured in watts (joules per second) or kilowatts. A 100-watt light bulb consumes 100 joules of energy per second. Running it for 10 hours consumes 1 kWh (kilowatt-hour) of energy. An analogy: energy is like the total distance of a journey while power is the speed — distance equals speed multiplied by time, just as energy equals power multiplied by time.

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