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Energy Conversion Calculator
Use this energy conversion calculator to convert a value between joules, kilojoules, megajoules, watt-hours, kilowatt-hours, calories, kilocalories, British thermal units, therms, foot-pounds, ergs, electronvolts, and other supported energy units. Enter a value, select the source unit and target unit, and read the converted result.
This calculator converts units of energy. It does not calculate the efficiency of a real energy-conversion process. For example, converting 1 kWh to joules is an exact unit conversion, while estimating how much electrical energy a motor produces from a given input also requires an efficiency value.
How to Use the Energy Converter
Enter the energy value you want to convert.
Select the unit of the entered value.
Select the unit for the result.
Read the converted value and keep an appropriate number of significant digits.
Unit symbols are case-sensitive. For example, kWh means kilowatt-hour, while kW means kilowatt and measures power rather than energy.
Energy Conversion Formula
Energy units can be converted through the joule, the SI unit of energy. If the source and target factors are expressed in joules per unit, use:
Target value = Source value × Source factor in J ÷ Target factor in J
For example, 1 kilowatt-hour equals 3,600,000 joules and 1 megajoule equals 1,000,000 joules. Therefore:
2.5 kWh × 3,600,000 J/kWh ÷ 1,000,000 J/MJ = 9 MJ
Common Energy Units and Conversion Factors
The following factors express one unit in joules. Values marked exact follow directly from SI definitions or fixed conversion definitions. Rounded values are shown with enough digits for typical calculator use.
| Unit | Symbol | Value in Joules | Typical Use |
|---|---|---|---|
| Joule | J | 1 J | SI unit of energy, work, and heat |
| Kilojoule | kJ | 1,000 J | Heat, food energy, and mechanical work |
| Megajoule | MJ | 1,000,000 J | Fuel, thermal, and industrial energy |
| Watt-hour | Wh | 3,600 J exactly | Electrical energy and battery capacity |
| Kilowatt-hour | kWh | 3,600,000 J exactly | Electrical energy consumption |
| Thermochemical calorie | calth | 4.184 J exactly | Heat and scientific calculations |
| Thermochemical kilocalorie | kcalth | 4,184 J exactly | Food energy and heat |
| International Table calorie | calIT | 4.1868 J exactly | Legacy heat calculations |
| International Table British thermal unit | BtuIT | 1,055.05585262 J exactly | Heating and cooling calculations |
| Thermochemical British thermal unit | Btuth | 1,054.35026444 J exactly | Thermochemical calculations |
| U.S. therm | therm | 105,480,400 J | Natural-gas energy |
| Foot-pound force | ft·lbf | 1.355817948 J | Mechanical work in U.S. customary units |
| Erg | erg | 0.0000001 J exactly | Centimetre-gram-second system |
| Electronvolt | eV | 1.602176634 × 10−19 J exactly | Atomic, nuclear, and particle physics |
| Ton of TNT | t TNT | 4.184 × 109 J | Large energy releases |
SI Prefixes for Joules
| Unit | Symbol | Joule Equivalent |
|---|---|---|
| Millijoule | mJ | 10−3 J |
| Microjoule | µJ | 10−6 J |
| Nanojoule | nJ | 10−9 J |
| Kilojoule | kJ | 103 J |
| Megajoule | MJ | 106 J |
| Gigajoule | GJ | 109 J |
| Terajoule | TJ | 1012 J |
Energy Conversion Examples
Convert Watt-Hours to Joules
A 500 Wh battery stores a nominal energy of:
500 Wh × 3,600 J/Wh = 1,800,000 J = 1.8 MJ
This energy rating does not by itself state the battery's usable energy, discharge limit, or conversion losses.
Convert Kilowatt-Hours to Kilocalories
Using the thermochemical kilocalorie:
2.5 kWh = 9,000,000 J ÷ 4,184 J/kcal = 2,151.05 kcal
Convert British Thermal Units to Kilowatt-Hours
Using the International Table Btu:
1,000 BtuIT = 1.05505585262 MJ = 0.29307107 kWh
Convert Food Calories to Megajoules
In nutrition, one capital-C Calorie is commonly treated as one kilocalorie:
500 Cal = 500 kcal × 4,184 J/kcal = 2.092 MJ
Convert Joules to Foot-Pounds
100 J ÷ 1.355817948 J/(ft·lbf) = 73.7562 ft·lbf
Convert Electronvolts to Joules
1 MeV = 106 eV = 1.602176634 × 10−13 J
Energy and Power Are Different Quantities
Energy is the capacity transferred or used in performing work or producing change. The SI unit is the joule. Power is the rate at which energy is transferred or converted. The SI unit is the watt, where 1 W = 1 J/s.
When power is constant, energy is calculated from:
E = P × t
where E is energy, P is power, and t is time. A 2 kW heater operating for 3 hours uses 6 kWh of energy. Do not enter a value in kW into an energy converter unless it has first been multiplied by an operating time.
Common Energy Formulas
A unit converter changes the way a known energy value is expressed. If energy must first be calculated from physical quantities, one of the following formulas may apply.
| Energy Type | Formula | Conditions and Variables |
|---|---|---|
| Mechanical work | E = Fd | Force F is constant and parallel to displacement d |
| Kinetic energy | E = ½mv2 | m is mass and v is speed |
| Gravitational potential energy | E = mgh | Near Earth's surface; m is mass, g is local gravitational acceleration, and h is height change |
| Constant electrical power | E = Pt = VIt | P is power, V is voltage, I is current, and t is time |
| Capacitor energy | E = ½CV2 | C is capacitance and V is voltage across the capacitor |
| Inductor energy | E = ½LI2 | L is inductance and I is current |
| Efficiency | η = Eout ÷ Ein | Multiply by 100% to express efficiency as a percentage |
Important Unit Conventions
Calorie and Calorie Are Not Always the Same
A lowercase calorie can refer to a small calorie, while a capital-C Calorie used in nutrition denotes a kilocalorie. The thermochemical calorie is exactly 4.184 J, so 1 thermochemical kilocalorie equals 4,184 J. Some older engineering references use the International Table calorie, which is exactly 4.1868 J. Select the definition that matches the source data.
BTU Definitions Differ Slightly
The International Table Btu and thermochemical Btu are not identical. One BtuIT is exactly 1,055.05585262 J, while one Btuth is exactly 1,054.35026444 J. When a document says only “BTU,” check its stated convention before making a high-precision conversion.
Watt-Hour Is an Energy Unit
A watt is a unit of power, but a watt-hour is a unit of energy because it multiplies power by time. The hour is converted to 3,600 seconds, giving 1 Wh = 3,600 J and 1 kWh = 3.6 MJ.
Battery Ah Is Not Energy Without Voltage
Ampere-hour measures electric charge, not energy. For an idealized constant-voltage estimate:
Energy in Wh = nominal voltage in V × charge capacity in Ah
Actual usable battery energy depends on the discharge profile, cutoff voltage, temperature, aging, and conversion efficiency.
Accuracy and Rounding
Do not report more significant digits than the original measurement supports.
Keep extra digits during intermediate calculations and round only the final result.
Choose the correct calorie, BTU, or therm convention when the distinction matters.
Use scientific notation for extremely large or small values.
Remember that a unit conversion does not add uncertainty, but rounded conversion factors can.
Common Energy Conversion Mistakes
Confusing kW, a power unit, with kWh, an energy unit.
Treating a nutritional Calorie as a small calorie instead of a kilocalorie.
Using a generic BTU value when the source specifies BtuIT or Btuth.
Converting battery ampere-hours directly to watt-hours without a voltage.
Assuming that equivalent unit values describe the efficiency of a real device.
Mixing hours and seconds in E = Pt.
Energy Conversion FAQ
What is the SI unit of energy?
The joule, symbol J, is the SI derived unit of energy, work, and quantity of heat. In SI base units, 1 J = 1 kg·m2/s2. It is also equal to 1 N·m and 1 W·s.
How many joules are in one kilowatt-hour?
There are exactly 3,600,000 joules in one kilowatt-hour. Therefore, 1 kWh = 3.6 MJ.
How many kilowatt-hours are in one megajoule?
One megajoule equals approximately 0.277777778 kWh.
How do I convert joules to kilowatt-hours?
Divide the value in joules by 3,600,000. For example, 7,200,000 J ÷ 3,600,000 = 2 kWh.
Is a food Calorie equal to a kilocalorie?
Yes. In nutrition, one Calorie with a capital C is commonly used for one kilocalorie, equal to 1,000 small calories. Under the thermochemical definition, 1 kcal = 4,184 J.
Can I convert watts directly to joules?
Not without a time interval. Since 1 W = 1 J/s, multiply power in watts by time in seconds to obtain energy in joules when power is constant.
Does converting energy units account for efficiency?
No. Unit conversion only expresses the same energy quantity in another unit. To estimate real output energy, multiply input energy by efficiency expressed as a decimal, provided the efficiency is known for the relevant operating conditions.
