Joules to Watts Calculator
Convert energy in joules to power in watts over any time — or watts back to joules — with kilojoules and watt-hours.
Watts are joules per second, so a conversion always needs the time.
Quick answer
Watts = joules ÷ seconds and joules = watts × seconds. One watt is one joule per second.
- 3,600 J over 1 hour = 1 W; over 1 minute = 60 W
- A 100 W bulb for 10 minutes = 100 × 600 = 60,000 J (60 kJ)
- 1 Wh = 3,600 J; 1 kWh = 3.6 MJ
How this conversion works
W = J ÷ s · J = W × s · Wh = J ÷ 3,600 · kJ = J ÷ 1,000.Joules and watts
The joule is the SI unit of energy; the watt is the SI unit of power, defined as one joule per second. Converting between them is therefore always a question of time: the same amount of energy is a small power if it is spread over an hour and a large one if it is delivered in a second. Electricity bills and batteries use watt-hours instead of joules because the numbers are smaller — one watt-hour is 3,600 joules.
Joules to watts examples
| Energy | Delivered over | Power |
|---|---|---|
| 1 J | 1 second | 1 W |
| 100 J | 1 second | 100 W |
| 1,000 J | 1 second | 1,000 W |
| 3,600 J | 1 hour | 1 W |
| 3,600 J | 1 minute | 60 W |
| 60,000 J | 1 minute | 1,000 W |
| 1,000,000 J | 1 hour | 277.8 W |
Working in watt-hours? Use the watts to watt-hours calculator, or kWh to MJ for large amounts of energy.
Frequently asked questions
Divide joules by the time in seconds. 3,600 J delivered over 60 seconds is 60 W; the same 3,600 J over an hour is 1 W.
A watt is one joule per second, so a 1 W device uses 1 J every second, 60 J a minute and 3,600 J an hour.
Multiply watts by seconds. A 100 W bulb running for 10 minutes (600 s) uses 60,000 J, or 60 kJ.
3,600. One watt sustained for one hour is 3,600 joules, so 1 kWh is 3.6 million joules (3.6 MJ).
No. Joules measure energy and watts measure power (energy per second), so the time over which the energy is delivered is always needed.
Electrical unit converters
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- Amps to watts calculator
- Watts to kilowatts calculator
- kW to kVA calculator
- Ohm's law calculator
- Ah to Wh calculator
- Ah to kWh calculator
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- kWh to mAh calculator
- Watts to watt-hours calculator
- HP to kW calculator
- BTU to kW calculator
- kWh to MJ calculator
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Learn more
Sources & standards
The joule, the watt and the relation 1 W = 1 J/s are SI definitions.
- NIST Special Publication 811 — Guide for the Use of the International System of Units — National Institute of Standards and Technology
Defines the joule, the watt and the watt-hour used on this page.