Supported units
- Joule (J)
- Kilojoule (kJ)
- Watt-hour (Wh)
- Kilowatt-hour (kWh)
- Calorie (cal)
- Kilocalorie (kcal)
Conversion tool
Energy Converter
Select a category, enter a value, and convert between compatible units.
Energy units explained
Energy represents the capacity to perform work or transfer heat. The joule is the SI unit, while kilowatt-hours and thermal energy units are common in practical applications.
Common engineering uses
Energy conversion is used in electrical systems, mechanical work, heating, fuel calculations and utility consumption.
Formula
E = P × t
Energy transferred equals power multiplied by time. When power is measured in watts and time in seconds, the result is expressed in joules because one watt equals one joule per second.
- E = energy, measured in joules (J)
- P = power, measured in watts (W)
- t = elapsed time, measured in seconds (s)
Common conversions
| From | Equivalent value |
|---|---|
| 1 kilojoule (kJ) | 1,000 joules (J) |
| 1 megajoule (MJ) | 1,000 kJ = 1,000,000 J |
| 1 watt-hour (Wh) | 3,600 joules (J) |
| 1 kilowatt-hour (kWh) | 3.6 megajoules (MJ) |
| 1 British thermal unit (BTU, IT) | approximately 1,055.06 joules (J) |
| 1 thermochemical calorie (cal) | 4.184 joules (J) |
Convert 2.5 kWh to MJ
- Use the relationship 1 kWh = 3.6 MJ.
- Multiply 2.5 by 3.6.
- 2.5 × 3.6 = 9 MJ.
2.5 kWh = 9 MJ
This conversion describes energy quantity. It does not include losses or equipment efficiency.
Engineering applications
- Comparing electrical consumption in kWh with engineering calculations in joules or megajoules.
- Calculating mechanical work and energy stored in moving or elevated components.
- Evaluating heating, cooling and thermal-process energy.
- Comparing batteries, fuels and other energy-storage systems.
- Estimating utility consumption and operating costs for equipment.
Common mistakes
- Confusing kilowatts with kilowatt-hours. kW measures power, while kWh measures energy.
- Forgetting that one watt-hour equals 3,600 joules rather than one joule.
- Mixing calorie or BTU definitions without identifying the reference standard used.
- Comparing input and useful output energy without allowing for system efficiency.
- Misreading prefixes such as kJ and MJ, which differ by a factor of 1,000.
Frequently asked questions
How many joules are in one kWh?
One kilowatt-hour equals exactly 3,600,000 joules, or 3.6 megajoules.
What is the difference between kW and kWh?
A kilowatt is a rate of energy transfer. A kilowatt-hour is the energy transferred by one kilowatt operating for one hour.
What is the SI unit of energy?
The SI unit is the joule (J).
Is a newton metre always a unit of energy?
A joule is dimensionally equal to a newton metre, but newton metre is also used for torque. The physical context determines the quantity.
Does an energy conversion include efficiency?
No. It changes only the unit. Efficiency must be applied separately when comparing input and useful output energy.