About the Joules to Watts Calculator
A joule is an amount of energy and a watt is a rate of using it: one watt is one joule every second. To convert joules to watts you therefore divide the energy by the time it took. This calculator does that for times in seconds, minutes or hours, and shows the answer in watts and kilowatts. It also converts the energy to kilowatt-hours, the unit on electricity bills. Use it for physics problems, for working out the power of a heater from the energy it delivered, or for finding the average power of an appliance from a meter reading.
Updated October 2026
How to use the Joules to Watts Calculator
- 1Enter the energy in joules.
- 2Enter the time taken.
- 3Choose seconds, minutes or hours.
- 4Read the power in watts and kilowatts.
- 5Check the energy in kilowatt-hours for billing comparisons.
More examples
A person climbing stairs might do 2,400 joules of work in 8 seconds, an average of 300 watts. A solar panel that collects 18 million joules over 5 hours produces an average of 1,000 watts. A phone charger that supplies 36,000 joules in 10 minutes is delivering 60 watts.
Averages can hide peaks. If an appliance uses 1,800 joules in 10 seconds, its average power is 180 watts, but it may have drawn 600 watts for a short burst and almost nothing the rest of the time. For motors and compressors that cycle on and off, a long measurement gives a better picture of the average, while a short one shows the peak. Energy meters that record total joules or kilowatt-hours over a day are the best source for this calculation.
Athletes and cyclists meet the same sum when they look at power meters. A rider who does 270,000 joules of work in 15 minutes averages 300 watts, which is a solid effort for a recreational cyclist. Divide by body mass to get watts per kilogram, the figure used to compare riders on climbs. The same principle applies to rowing machines, treadmills and any equipment that reports total work done and the time it took.
Common conversions
- 1 W = 1 J/s.
- 1 kW = 1,000 J/s.
- 1 Wh = 3,600 J.
- 1 kWh = 3,600,000 J.
- 1 horsepower is about 746 W.
Useful related tools
The Energy Converter changes joules into calories and kilowatt-hours, the Power Converter changes between watts and horsepower, the kWh Calculator multiplies power by time and the Energy Cost Calculator turns the kilowatt-hours into money.
Features
- Power from joules and time
- Seconds, minutes and hours
- Kilowatts and kilowatt-hours
- Rejects zero or negative time
- Quick and private
- Great for homework and checks
Frequently asked questions
What is the formula for joules to watts?
Watts equal joules divided by seconds. If a device uses 5,400 joules in 30 seconds, its power is 180 watts.
Why does time matter?
Energy is the total amount used, and power is how fast it is used. The same 3,600 joules could be 1 watt for an hour or 3,600 watts for a second, so you must know the time.
How do I convert joules to kilowatt-hours?
Divide by 3,600,000, because a kilowatt-hour is 3.6 million joules. A 7.2 million joule heater output is 2 kilowatt-hours.
How do I convert minutes or hours to seconds?
Multiply minutes by 60 and hours by 3,600. The calculator has a time unit drop-down, so you can enter minutes or hours directly.
Is a watt the same as a joule per second?
Yes, by definition. It is named after James Watt. A 60 watt bulb transfers 60 joules of energy every second as light and heat.
Can I use this to find the power of a heater?
Yes. If a kettle adds 336,000 joules to water in 2 minutes, its power is 336,000 divided by 120, which is 2,800 watts. Real kettles lose some heat, so the electrical power is a bit higher than the heat delivered.