Convert energy values between joules, kilojoules, watt-hours, kilowatt-hours, calories, British thermal units, electronvolts, and other supported units.
Enter an energy value, select its current unit, and choose the target unit to calculate the equivalent amount.
What Is Energy?
Energy is a physical quantity associated with the ability of a system to perform work, transfer heat, produce radiation, or cause another measurable change. Energy can be stored or transferred in mechanical, thermal, electrical, chemical, nuclear, and electromagnetic forms.
Although these forms may appear different, they can be compared after being expressed in a common energy unit. The SI unit of energy is the joule (J).
1 J = 1 N·m = 1 kg·m²/s²
How Energy Conversion Works
This converter uses the joule as its common reference unit. The source value is first converted to joules and then converted from joules to the selected target unit.
Target value = original value × source-unit joule factor ÷ target-unit joule factor
For example, one watt-hour equals 3,600 joules. Therefore, converting 5 Wh to joules gives:
5 Wh × 3,600 = 18,000 J
Common Energy Units
Different fields use different energy units according to the scale and purpose of the measurement:
- Joule (J) — the SI unit used in science and engineering.
- Pound,Kilojoule (kJ) — equal to 1,000 joules and commonly used for food and thermal energy.
- Calorie (cal) — a small heat-energy unit.
- Kilocalorie (kcal) — equal to 1,000 calories and commonly used for food energy.
Common Energy Conversion Factors
- 1 kJ = 1,000 J
- 1 MJ = 1,000,000 J
- 1 Wh = 3,600 J
- 1 kWh = 3,600,000 J
- 1 thermochemical cal = 4.184 J
- 1 thermochemical kcal = 4,184 J
- 1 eV = 1.602176634 × 10−19 J
- 1 erg = 0.0000001 J
- 1 ft·lbf ≈ 1.3558179483 J
Energy, Power, and Time
Energy and power are related, but they are not the same quantity. Energy measures a total amount transferred or used, while power measures the rate of energy transfer.
Energy = Power × Time
A 2 kW heater operating for three hours uses:
2 kW × 3 h = 6 kWh
Because time is required, kilowatts cannot be converted directly to kilowatt-hours without additional information.
Types of Energy
- Kinetic energy is associated with motion.
- Potential energy is associated with position or configuration.
- Thermal energy is connected with the microscopic motion and interactions of particles.
- Chemical energy is associated with chemical bonds and reactions.
- Electrical energy is transferred or stored through electric charges and fields.
- Radiant energy is carried by electromagnetic radiation.
- Nuclear energy is associated with changes involving atomic nuclei.
Energy may change from one form to another. A battery, for example, can transform stored chemical energy into electrical energy, which a lamp then converts partly into light and partly into heat.
When to Convert Energy Units
Energy conversions are used when comparing electricity bills, battery capacities, appliance consumption, heating systems, fuel values, food labels, laboratory measurements, and engineering calculations.
A household electricity bill may report energy in kilowatt-hours, while an engineering calculation uses joules. Food packaging may use kilojoules and kilocalories, while heating equipment may be rated using BTUs.
Converting all values to the same unit makes comparisons possible and reduces the risk of mixing quantities that use different scales or definitions.
Energy Conversion FAQ
What is the SI unit of energy?
The joule, with the symbol J, is the SI-derived unit of energy. One joule is the work performed when a force of one newton moves its point of application one meter in the force’s direction.
What is the difference between energy and power?
Energy is the total amount transferred or used. Power is the rate at which that transfer occurs. Joules and kilowatt-hours measure energy, while watts and kilowatts measure power.
What is the difference between a watt and a watt-hour?
A watt measures power. A watt-hour measures energy and represents one watt of power sustained for one hour.
Can kilowatts be converted directly to kilowatt-hours?
No. The operating time must be known. Multiply kilowatts by hours to obtain kilowatt-hours.
How many joules are in one kilowatt-hour?
One kilowatt-hour equals exactly 3,600,000 joules. This follows from 1 kW = 1,000 J/s and 1 hour = 3,600 seconds.
How do I calculate the energy used by an appliance?
Multiply the appliance’s power by its operating time. A 1.5 kW appliance operating for four hours uses 6 kWh.
What is the difference between a calorie and a food Calorie?
A lowercase calorie, written cal, is a small energy unit. A food Calorie normally means one kilocalorie: 1 Calorie = 1 kcal = 1,000 cal.
Why do food labels show both kilojoules and kilocalories?
Kilojoules are metric energy units, while kilocalories remain familiar in nutrition. Both values describe the same food energy in different units.
What is a British thermal unit?
A British thermal unit is an energy unit used in heating, cooling, and fuel measurements. Several historical BTU definitions exist, so the specific variant matters in precision calculations.
What is an electronvolt?
An electronvolt is the energy gained or lost by an elementary charge moving through an electric potential difference of one volt. One electronvolt equals exactly 1.602176634 × 10−19 J.
Is a foot-pound of energy the same as pound-foot torque?
No. Energy and torque have equivalent dimensional components but describe different physical quantities. The surrounding physical context determines whether a value represents work or rotational effect.
Can different forms of energy be converted using the same unit converter?
Yes, if the values represent quantities of energy. A joule of thermal energy and a joule of mechanical energy are equal in magnitude, even though their physical forms differ.
What does energy efficiency mean?
Efficiency compares useful output energy with input energy:
Efficiency = useful output energy ÷ input energy × 100%
An efficiency below 100% indicates that some input energy was transferred into other forms, often including unwanted heat or sound.
Why might two converters show slightly different energy results?
The converters may round differently or use different variants of units such as the calorie or BTU. Check the unit definition and conversion factor when precision is important.
Are all energy conversion factors exact?
No. Some relationships are exact by definition, including 1 Wh = 3,600 J and 1 thermochemical calorie = 4.184 J. Other factors may be approximate or depend on the selected historical definition.
Does converting an energy value change the physical amount?
No. Conversion changes only the unit and numerical representation. For example, 1 kWh and 3.6 MJ represent the same amount of energy.
Can energy be created or destroyed?
In an isolated system, total energy is conserved. Energy can be transferred between objects or transformed from one form to another, but the total amount remains constant.