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kW to kJ Converter
Turn kilowatts and a time period into kilojoules — with reverse kJ → kW mode, a power-rate converter (kJ/h, BTU/h, kcal/h), a kJ-centered energy converter, hand-checked worked examples, and every practical equivalent.
Kilowatts → Kilojoules calculator
Enter power and how long it runs. Energy in kilojoules is P × t: with time in seconds it's direct; with hours, multiply by 3,600.
ENERGY
7,200
kilojoules (kJ)
Also equals
One-tap presets
Power-rate converter
Convert between power units: kW, watts, horsepower — and energy-per-time rates like kJ/h, BTU/h and kcal/h. Remember: 1 kW = 3,600 kJ/h exactly.
Convert any energy unit to any other — anchored on the kilojoule. 1 kWh = 3,600 kJ exactly.
Worked examples ✓ hand-verified
Each example below was computed by hand and cross-checked against the calculator engine — the numbers match exactly.
✓ Verified
Example 1 — 2 kW heater for 1 hour
Time in seconds: 1 h = 3,600 s.
E = 2 kW × 3,600 s = 7,200 kJ.
On your electricity bill that's 2 kWh; as heat it's 6,824.3 BTU or 1,720.8 kcal.
2 × 3,600 = 7,200 kJ ✓
✓ Verified
Example 2 — 1.2 kW microwave for 3 minutes
Time in seconds: 3 min = 180 s.
E = 1.2 kW × 180 s = 216 kJ.
That's 0.06 kWh — about 204.7 BTU of heat into your food.
1.2 × 180 = 216 kJ ✓
✓ Verified
Example 3 — Reverse: 500 kJ over 10 minutes
Time in seconds: 10 min = 600 s.
P = 500 kJ ÷ 600 s = 0.8333 kW average power.
Use the kJ → kW mode above for any reverse calculation.
500 ÷ 600 = 0.8333 kW ✓
✓ Verified
Example 4 — Heat pump, 1 kW in, COP 4, 1 hour
Heat delivered = 1 kW × COP 4 = 4 kW of heat.
E = 4 kW × 3,600 s = 14,400 kJ of heat.
Electricity used: only 3,600 kJ (1 kWh) — the rest is moved, not made.
4 × 3,600 = 14,400 kJ heat ✓
Methodology & honest limitations
// kW → kJ (time in seconds)
E(kJ) = P(kW) × t(s)
// kW → kJ (time in hours — the practical shortcut)
E(kJ) = P(kW) × t(h) × 3,600
// kJ → kW (reverse)
P(kW) = E(kJ) ÷ t(s)
Constants used (exact definitions)
Relation
Value
Source
1 kW in kJ/h
3,600 (exact)
3,600 s per hour
1 kW in kJ/s
1 (exact)
1 W = 1 J/s, SI definition
1 kW in BTU(IT)/h
3,412.1416
1 BTU(IT) = 1,055.05585262 J
1 kW in kcal(th)/h
860.4207
1 cal(th) = 4.184 J (exact)
1 kWh in kJ
3,600 (exact)
SI definition
1 hp (mechanical) in W
745.699872
definition
Honest limitations. This calculator assumes constant power over the whole time period. Real appliances cycle on and off (a 2 kW heater with a thermostat may average far less), motors have startup surges, and heat-pump COP changes with outdoor temperature. Results are idealised physics, not meter readings. Negative inputs are rejected; a zero time period in reverse mode is invalid (division by zero).
Scope boundary — which tool do you need? This page is the kilojoule-centered power×time calculator. Need the same math in joules? Use our kW to joules converter. Need a broad 12-unit energy reference (therm, toe, eV, fuel equivalents)? See the energy unit converter hub. Want to price kWh against your electricity tariff? That's the energy cost calculator. Converting amps/volts to kW first? Start at the amps to kW calculator.
Frequently asked questions
What is the formula to convert kW to kJ?
Energy in kilojoules equals power in kilowatts times time in seconds: E(kJ) = P(kW) × t(s). If your time is in hours, multiply by 3,600 instead: E(kJ) = P(kW) × t(h) × 3,600. Example: 2 kW × 1 h × 3,600 = 7,200 kJ.
How many kJ are in 1 kW?
That depends on time — 1 kW is a rate, not an amount. 1 kW running for 1 second delivers 1 kJ; for 1 minute it delivers 60 kJ; for 1 hour it delivers 3,600 kJ. Without a time period there is no single answer, because kilowatts and kilojoules measure different quantities.
How do I convert kJ back to kW?
Divide the energy by the time in seconds: P(kW) = E(kJ) ÷ t(s). For example, 500 kJ delivered over 10 minutes (600 s) means 500 ÷ 600 = 0.833 kW average power. Use the reverse mode in the calculator above.
What does kW to kJ per hour mean?
kJ/h is a power unit (energy per time), not an energy amount. 1 kW = 3,600 kJ/h exactly, because 1 kW delivers 1 kJ every second, and there are 3,600 seconds in an hour. So a 2 kW heater has a rating of 7,200 kJ/h.
How many BTU per hour is 1 kW?
1 kW = 3,412.14 BTU(IT)/h (also written BTU/hr). The conversion uses the International Table BTU: 1 BTU(IT)/h = 1,055.05585262 J ÷ 3,600 s = 0.29307 W, so 1,000 W ÷ 0.29307 W = 3,412.14 BTU/h. The rate converter above computes this for any power.
How many kcal per hour is 1 kW?
1 kW = 860.42 kcal(th)/h (thermochemical calories). This comes from 1 kcal(th) = 4,184 J exactly: 3,600,000 J/h ÷ 4,184 J/kcal = 860.42 kcal/h. Food-label calories are kilocalories, so this is also the figure used for exercise-energy estimates.
How much energy does a 2 kW heater use in one hour?
7,200 kJ. Calculation: 2 kW × 1 h × 3,600 = 7,200 kJ, which equals 2 kWh (kilowatt-hours) of electricity on your bill, 6,824.3 BTU, or 1,720.8 kcal of heat. Try the preset button above to see every equivalent.
kW vs kWh vs kJ — what is the difference?
Explained simply: kW (kilowatt) is power — the rate energy flows. kWh (kilowatt-hour) and kJ (kilojoule) are both energy — the total amount. 1 kWh = 3,600 kJ exactly. Think of kW as the speed of water flowing from a tap, and kJ/kWh as the volume collected in the bucket.
Can I convert kW to kJ without knowing the time?
No. A kilowatt is energy per second and a kilojoule is an amount of energy — converting one to the other requires a duration. Any page that gives you a single 'kW to kJ' number without asking for time is wrong; you can only compute the energy from power × time.
How does heat pump COP relate to kilojoules?
COP multiplies the input power: heat delivered = kW in × COP. A 1 kW heat pump with COP 4 moves 4 kW of heat, so in one hour it delivers 4 × 3,600 = 14,400 kJ of heat while consuming only 3,600 kJ (1 kWh) of electricity. The worked example section above shows this step by step.
How do I convert kWh to kJ?
Multiply by 3,600: E(kJ) = E(kWh) × 3,600, exactly. So 2 kWh = 7,200 kJ and 0.5 kWh = 1,800 kJ. The energy converter panel above does this both ways.
Is 1 kJ/s the same as 1 kW?
Yes, exactly. A watt is defined as one joule per second, so 1 kJ/s = 1,000 J/s = 1,000 W = 1 kW. The rate converter treats kJ/s, kJ/min (×60), and kJ/h (×3,600) as power units.
How many megajoules are in 7,200 kJ?
7.2 MJ — divide kilojoules by 1,000. In general: J → kJ → MJ → GJ each step is a factor of 1,000. The energy converter panel converts between all of them instantly.
Why do some sites say you cannot convert kW to kJ?
They mean you cannot convert the units directly, because kW (power) and kJ (energy) have different physical dimensions — like speed vs distance. But you can always calculate the energy from power × time, which is exactly what this calculator does. The 'cannot convert' warning is about missing information (time), not about the math being impossible.
How is this different from the kW-to-joules converter and the energy unit converter?
This page is centered on the kilojoule as the working unit for power×time problems (heaters, heat pumps, appliances). The kW-to-joules tool targets the joule instead; the general energy unit converter is a broad 12-unit reference hub; the energy cost calculator prices kWh against electricity tariffs. Each answers a different intent — this one is for 'how much energy does X kW for Y time produce, in kJ and its practical equivalents'.
Is my data sent anywhere when I use this calculator?
No. This tool is 100% client-side: every calculation runs in your browser's JavaScript engine, no request is ever made to a server, and nothing you type leaves your device. You can verify this with your browser's network inspector.
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