kJ to kW Converter
You cannot turn kJ into kW with a single number. Energy needs a time, and the sum is power equals energy divided by time.
Quick answer: divide the kilojoules by the seconds. For example, 7,200 kJ in 3,600 s is 2 kW, 5,000 kJ in 100 s is 50 kW and 1,000 kJ in 60 s is 16.67 kW.
The kJ to kW formula
A watt is one joule per second, so a kilowatt is one kilojoule per second (Wikipedia: Watt).
Power in kW equals the energy in kJ divided by the time in seconds.
Take 7,200 kJ over 3,600 s. Divide to get 2 kW.
Take 5,000 kJ over 100 s. Divide to get 50 kW.
Energy needs time
A watt is a joule per second. It measures how fast energy moves (Wikipedia: Watt).
Energy equals power times time. A 100 W bulb on for one hour uses 360 kJ (Wikipedia: Watt).
A kW is a rate. A kJ is an amount. That is why you need the time.
How to calculate kJ to kW step by step
- Write down the kilojoules.
- Write down the time in seconds.
- Divide the kJ by the seconds.
- Add the unit kW.
Example 1: 7,200 kJ in 1 hour
- Start with 7,200 kJ over 3,600 s.
- 7,200 ÷ 3,600 = 2.
- The power is 2 kW.
7,200 kJ in 3,600 s = 2 kW
Example 2: 5,000 kJ in 100 s
- Start with 5,000 kJ over 100 s.
- 5,000 ÷ 100 = 50.
- The power is 50 kW.
5,000 kJ in 100 s = 50 kW
Example 3: 360 kJ in 1 hour
- A 100 W bulb uses 360 kJ in one hour.
- 360 ÷ 3,600 = 0.1.
- The power is 0.1 kW, which is 100 W.
360 kJ in 3,600 s = 0.1 kW
A 2 kW heater is about 2.68 horsepower, since 1 kW is about 1.34 hp (Wikipedia: Watt).
kJ to kW chart
| Energy | Time | Power |
|---|---|---|
| 7,200 kJ | 3,600 s | 2 kW |
| 3,600 kJ | 3,600 s | 1 kW |
| 1,000 kJ | 60 s | 16.6667 kW |
| 500 kJ | 10 s | 50 kW |
| 5,000 kJ | 100 s | 50 kW |
| 100 kJ | 1 s | 100 kW |
| 360 kJ | 3,600 s | 0.1 kW |
| 36,000 kJ | 3,600 s | 10 kW |
| 180,000 kJ | 3,600 s | 50 kW |
The highlighted row, 7,200 kJ in 3,600 s, is the default of the calculator. Change the time and the power changes with it.
A quick mental shortcut for kJ to kW
If the time is 60 seconds, divide the kJ by 60. So 600 kJ in a minute is 10 kW.
If the time is one hour, divide the kJ by 3,600.
What a kilojoule is
A kilojoule is 1,000 joules. It is a unit of energy (Wikipedia: Joule).
It tells you how much, not how fast.
What a kilowatt is
A kilowatt is 1,000 watts. It is a unit of power, which is how fast energy is used (Wikipedia: Watt).
One kW is one kJ every second.
Power versus energy
Power is the speed and energy is the amount. A car doing 60 km each hour has a speed, while the distance is the amount.
Power is to energy what speed is to distance.
Typical power values
A small electric heater uses about 1 kW. A PC power supply is often a few hundred watts. A large power plant makes around 1 GW (Wikipedia: Watt).
The average US home uses about 1 kW over a day.
Time in the sum
The time must be in seconds. If you have minutes, multiply by 60. If you have hours, multiply by 3,600.
So 1,800 kJ in 30 minutes is 1,800 kJ over 1,800 s, which is 1 kW.
kW is not kWh
A kWh is energy. It equals 3,600 kJ (Wikipedia: Kilowatt-hour). A kW is power.
Do not write kW/h for energy.
Horsepower
One kW is about 1.34 horsepower (Wikipedia: Watt). So 50 kW is about 67 hp.
Engines are often rated in both.
kW back to kJ
To go back, multiply the kW by the seconds. So 2 kW over 3,600 s is 7,200 kJ.
Use the kJ to kWh converter for the next step.
Check your answer
Multiply your kW answer by the seconds. You should get your starting kJ.
A bigger time always gives a smaller power for the same energy.
Quick answers
| Input | Result |
|---|---|
| 100 kJ in 1 s | 100 kW |
| 500 kJ in 10 s | 50 kW |
| 1,000 kJ in 60 s | 16.67 kW |
| 3,600 kJ in 3,600 s | 1 kW |
| 7,200 kJ in 3,600 s | 2 kW |
| 36,000 kJ in 3,600 s | 10 kW |
Where this calculator is used
Engineers find the power of a machine from its energy and time.
Home owners check how fast a heater uses energy.
Students solve physics sums on power.
Common mistakes to avoid
- Forgetting the time. You cannot get kW from kJ alone.
- Using hours instead of seconds. Convert the time first.
- Mixing up kW and kWh. One is power, one is energy.
- Multiplying instead of dividing. Power is energy over time.
Related calculators
A heater example
A heater gives out 1,800 kJ in 15 minutes. That is 900 s. So 1,800 ÷ 900 gives 2 kW.
It is a 2 kW heater.
A machine example
A machine does 90,000 kJ of work in 10 minutes. That is 600 s. So 90,000 ÷ 600 gives 150 kW.
That is about 201 hp.
Finding the energy instead
If you know the kW and the time, multiply them. A 3 kW kettle running for 120 s uses 360 kJ.
That is 0.1 kWh.
Frequently asked questions
How do I convert kJ to kW?
Divide the kilojoules by the time in seconds.
Can I convert kJ to kW without time?
No. Energy needs a time to give power.
How many kW is 7,200 kJ in one hour?
2 kW.
What is one kW in kJ per second?
1 kJ per second.
What is the difference between kW and kWh?
A kW is power. A kWh is energy.
How many kJ is one kWh?
3,600 kJ.
Sources
- Wikipedia: Watt (en.wikipedia.org)
- Wikipedia: Joule (en.wikipedia.org)
- Wikipedia: Kilowatt-hour (en.wikipedia.org)
