kJ = kW × seconds
No conversion constant — the SI units already line up.
A watt is defined as one joule per second, so a kilowatt delivers exactly one kilojoule every second. A 10 kW element held for 60 seconds delivers 600 kJ.
kW to kJ
Convert power into kilojoules for short-duration processes measured in seconds rather than hours.
Need the power figure before the energy figure? Work it out with the kVA to kW converter first.
kW to kJ Calculator
kJ = kW × seconds10 kW
kW
60 s
s
Quick values
At a glance
kJ = kW × seconds
- Energy
- 600 kJ
- Power
- 10 kW
- Duration
- 60 s
Every diagram and table on this page updates with the calculator above.
Output
Live600kJ
kJ = 10 kW × 60 s = 600 kJ
In megajoules0.6 MJ
In joules600,000 J
The kW to kJ Formula
The neatest conversion in electrical engineering: one kilowatt is one kilojoule per second.
The conversion
LiveInput10kW
Result600kJ
Why Process Engineering Works in Seconds
Kilowatt-hours suit billing periods. Kilojoules suit events: a weld pass, a press stroke, an induction heating cycle, a capacitor discharge. At those timescales kWh figures become unreadable fractions.
- 1 kJ
- One kilowatt for one second, or 100 W for ten seconds.
- 3,600 kJ
- One kilowatt-hour. The bridge between the two worlds.
- 1 MJ
- A thousand kilojoules — roughly the energy in a spoonful of diesel.
Energy in a burst
LiveEnergy delivered600 kJ
Heat Input in Welding
Welding procedures specify heat input in kilojoules per millimetre of weld. Too much coarsens the grain structure and softens the heat-affected zone; too little risks lack of fusion and hydrogen cracking.
kJ/mm = (V × A × efficiency) ÷ (travel speed in mm/s × 1,000)
Process efficiency: about 0.8 for MMA, 0.8 for MIG, 0.6 for TIG.
A MIG pass on structural steel
Given
- Arc voltage: 28 V
- Current: 280 A
- Travel speed: 5 mm/s
- Efficiency: 0.8
Working
- Arc power = 28 × 280 = 7,840 W = 7.84 kW
- Effective = 7.84 × 0.8 = 6.27 kW
- Heat input = 6.27 ÷ 5 = 1.25 kJ/mm
About 1.25 kJ/mm, comfortably inside the typical 0.5–2.5 kJ/mm window for structural steel.
Heat input is a qualified variable
On coded work, heat input outside the range in the welding procedure specification invalidates the qualification, regardless of how the weld looks.
Energy against duration
Live60 s600 kJ
kW to kJ Conversion Chart
Your power level held for a range of durations.
Conversion chart
Live| s | Energy (kJ) | Energy (MJ) |
|---|---|---|
| 1 s | 10 | 0.01 |
| 2 s | 20 | 0.02 |
| 5 s | 50 | 0.05 |
| 10 s | 100 | 0.1 |
| 15 s | 150 | 0.15 |
| 30 s | 300 | 0.3 |
| 60 s | 600 | 0.6 |
| 120 s | 1,200 | 1.2 |
| 300 s | 3,000 | 3 |
| 600 s | 6,000 | 6 |
| 900 s | 9,000 | 9 |
| 1,800 s | 18,000 | 18 |
| 3,600 s | 36,000 | 36 |
Energy for Every Power and Duration
Handy for process design, where both the power available and the cycle time are usually adjustable.
kJ lookup grid
Live| kW | 1 s | 10 s | 30 s | 60 s | 300 s |
|---|---|---|---|---|---|
| 1 kW | 1 | 10 | 30 | 60 | 300 |
| 5 kW | 5 | 50 | 150 | 300 | 1,500 |
| 10 kW | 10 | 100 | 300 | 600 | 3,000 |
| 25 kW | 25 | 250 | 750 | 1,500 | 7,500 |
| 50 kW | 50 | 500 | 1,500 | 3,000 | 15,000 |
| 100 kW | 100 | 1,000 | 3,000 | 6,000 | 30,000 |
| 250 kW | 250 | 2,500 | 7,500 | 15,000 | 75,000 |
How Much Will It Actually Heat?
Kilojoules become tangible once you connect them to a temperature rise. The specific heat capacity of the material tells you how many kilojoules each kilogram needs per degree.
kJ = mass (kg) × specific heat (kJ/kg·K) × ΔT (K)
Water is 4.18 kJ/kg·K; steel is about 0.49; aluminium 0.90.
Heating 20 litres of water by 40 °C
Given
- Mass: 20 kg
- Specific heat: 4.18 kJ/kg·K
- Temperature rise: 40 K
Working
- Energy = 20 × 4.18 × 40
- Energy = 3,344 kJ
About 3,344 kJ — a 10 kW element would need roughly 5.6 minutes, ignoring losses.
Power and energy
LiveEvery Power Converter in One Place
29 calculators built on the same maths as the tool above. Each one has its own inputs, interactive diagrams and worked examples.
kW to kVA
kVA = kW ÷ PFkVA to MW
MW = kVA × PF ÷ 1,000kVA to MVA
MVA = kVA ÷ 1,000VA to kVA
kVA = VA ÷ 1,000VA to kW
kW = VA × PF ÷ 1,000kVA to Watts
W = kVA × 1,000 × PFkW to Watts
W = kW × 1,000kW to GW
GW = kW ÷ 1,000,000kVA to kW (Single Phase)
kW = kVA × PFkVA to Amps
A = kVA × 1,000 ÷ (V × √3 for 3-phase)kVA to Amps (Three Phase)
A = kVA × 1,000 ÷ (√3 × V)Amps to kW
kW = √3 × V × A × PF ÷ 1,000kW to Volts
V = kW × 1,000 ÷ (√3 × A × PF)kW to kWh
kWh = kW × hourskW to MWh
MWh = kW × hours ÷ 1,000kW to GJ
GJ = kW × hours × 0.0036Current
kW to kJ
kJ = kW × secondskW to Joules
J = kW × 1,000 × secondskW to HP
hp = kW × 1.34102kW to BHP
bhp = kW × 1.34102kW to Nm
N·m = 9549.3 × kW ÷ rpmkW to Torque
T = 9549.3 × kW ÷ rpmkW to RPM
rpm = 9549.3 × kW ÷ N·mkW to CC
cc = (kW ÷ kW per litre) × 1,000kW to BTU/hr
BTU/hr = kW × 3,412.142kW to Ton
TR = kW ÷ 3.51685kW to GPM
GPM = BTU/hr ÷ (500 × ΔT)kW to CFM
CFM = BTU/hr ÷ (1.08 × ΔT)kW to PSI
bar = kW × η × 600 ÷ L/minkW to kJ Questions
Common questions about converting kw to kj.
A kilowatt delivers 1 kJ every second, so 1 kW for 1 second is 1 kJ and 1 kW for an hour is 3,600 kJ.
Multiply power by the duration in seconds: kJ = kW × seconds.
3,600 kJ, which is 3.6 MJ.
Kilojoules suit processes that last seconds or minutes, where kilowatt-hours would be tiny fractions. Both measure the same physical quantity.
Arc power multiplied by time per unit length, normally expressed in kJ/mm. It is calculated from voltage, current, travel speed and process efficiency.
About 335 kJ to raise it from 20 °C to 100 °C, plus roughly 2,260 kJ more if you actually want to evaporate it.