Joules to Watts Converter
Enter an amount of energy and the time it is delivered or used over, and the converter returns the average power in milliwatts, watts, kilowatts or megawatts. A joule has no wattage of its own: power always needs a duration.
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Created by the Reeliy Editorial Team. Last updated: Oct 19, 2026
Why joules cannot be turned into watts on their own
The joule (J) measures an amount of energy, the same way a liter measures an amount of water. The watt (W) measures a rate: how quickly that energy is being delivered or used. By definition, one watt is one joule per second.
That is why a question such as "how many watts is 500 joules?" has no single answer. Five hundred joules delivered in one second is 500 W; the same 500 joules spread over ten seconds is 50 W. The converter therefore asks for two numbers, an energy and a time, and works out the average power over that time.
The joules to watts formula
power (W) = energy (J) ÷ time (s)Equivalent to P = E ÷ t.
Before dividing, bring everything to joules and seconds, then scale the answer to the unit you want:
- Energy: kilojoules × 1,000 and megajoules × 1,000,000 give joules.
- Time: minutes × 60 and hours × 3,600 give seconds.
- Power: watts × 1,000 give milliwatts, watts ÷ 1,000 give kilowatts, and watts ÷ 1,000,000 give megawatts.
How to use the converter
- Type the energy and pick its unit: joules, kilojoules or megajoules.
- Type the time and pick whether it is per second, per minute or per hour.
- Choose the power unit you want. The result updates as you type.
The answer is shown to as many as 16 decimal places with trailing zeros removed, and it stays blank if the time is zero, because dividing by zero has no meaning.
Worked examples
Example 1. A small heater element gives off 5,400 J over 90 seconds. Power = 5,400 ÷ 90 = 60 W.
Example 2. An appliance uses 3.6 MJ in one hour. In joules that is 3,600,000 J, and one hour is 3,600 s, so power = 3,600,000 ÷ 3,600 = 1,000 W, or 1 kW. This is also why one kilowatt-hour equals 3.6 megajoules.
Example 3. A battery pack delivers 200 kJ in 2 minutes. That is 200,000 J in 120 s, so power = 200,000 ÷ 120 ≈ 1,666.67 W, or about 1.667 kW.
Joules per second to watts table
| Energy per second (J/s) | Watts (W) | Kilowatts (kW) |
|---|---|---|
| 1 | 1 | 0.001 |
| 10 | 10 | 0.01 |
| 60 | 60 | 0.06 |
| 100 | 100 | 0.1 |
| 500 | 500 | 0.5 |
| 1,000 | 1,000 | 1 |
| 2,500 | 2,500 | 2.5 |
| 5,000 | 5,000 | 5 |
| 10,000 | 10,000 | 10 |
| Energy used per hour | Average power (W) |
|---|---|
| 1 kJ | 0.2778 |
| 10 kJ | 2.7778 |
| 100 kJ | 27.7778 |
| 360 kJ | 100 |
| 1,000 kJ (1 MJ) | 277.7778 |
| 3,600 kJ (3.6 MJ) | 1,000 |
Average power and common slips
The converter returns average power over the time you entered. A device that spikes and idles can draw far more than its average at any given moment, so use peak ratings when sizing wiring or a power supply.
Watch the capitals. The menu lists MilliWatts (mW, a thousandth of a watt) and MegaWatts (MW, a million watts). They differ by a factor of one billion.
Another frequent mix-up is between kilowatts (power) and kilowatt-hours (energy). A kilowatt-hour is the energy used by a 1 kW load running for an hour, which is 3.6 MJ.
Related tools
To go the other way, from power and time to energy, use the watts to joules converter. For electrical loads, the Ohm's law calculator links voltage, current, resistance and power, and the kinetic energy calculator gives the energy of a moving mass in joules. More unit tools are listed under physics converters.
Frequently asked questions
How many watts is one joule?
A joule is an amount of energy and a watt is a rate, so there is no fixed answer. One joule delivered in one second is 1 W; one joule delivered over two seconds is 0.5 W.
Is a joule per second the same as a watt?
Yes. The watt is defined as one joule per second, so converting J/s to W only changes the name, never the number.
How do I convert kilojoules to kilowatts?
Divide the kilojoules by the number of seconds. Kilojoules per second are kilowatts, so 500 kJ delivered in 100 s is 5 kW.
How many joules are in a kilowatt-hour?
A kilowatt-hour is 3,600,000 joules (3.6 MJ), because 1,000 W running for 3,600 s transfers 1,000 × 3,600 joules.
References
Cite this converter and page
If you use this page in your work, you can cite it as:
Reeliy Editorial Team. (2026). Joules to Watts Converter. Reeliy. https://reeliy.com/converters/convert-joules-to-watts“Joules to Watts Converter.” Reeliy, 2026, https://reeliy.com/converters/convert-joules-to-watts.