What next?
Power is a rate. These are the quantities either side of it.
How to use this calculator
- Enter the work or energy in joules.
- Enter the time taken in seconds.
- Read the power in watts.
What your result means
Power is the rate of doing work — energy divided by time, in watts. The same job done faster demands more power, and one watt is simply one joule per second.
Why this one is different
Zero seconds returns no figure and says why — power over no elapsed time is undefined, not infinite. When there is a time, the rate is also converted into the kilowatt-hours it would use if held for an hour, which is the unit an electricity bill is written in, next to watts, kilowatts and horsepower.
The difference between strong and fast
Power isn’t about how much work you do — it’s how quickly you do it. Two engines can shift the same load, but the more powerful one does it faster. That’s the whole idea behind horsepower, a term James Watt coined to sell steam engines to horse owners.
From light bulbs to car specs to your electricity bill, power in watts is the number that decides speed, brightness and running cost.
How it works
Power measures how quickly work is done or energy is transferred. Doing the same job in less time takes more power. One watt is one joule per second.
Formula
Worked example
A hoist doing 4,500 J of work in 9 seconds:
P = 4,500 ÷ 9
P = 500 W (0.67 hp)
Halving the time to 4.5 s doubles the power required to 1,000 W — the work is the same, the rate is not.
Frequently asked questions
How many watts in one horsepower?＋
One mechanical horsepower is about 745.7 watts, the unit used here.
What's the difference from energy?＋
Energy (joules) is the total amount of work; power (watts) is how fast it's delivered.
How do watts relate to the kWh on my energy bill?＋
A kilowatt-hour is a thousand watts sustained for one hour, so it is a quantity of energy rather than a rate. A 500 W appliance running two hours uses 1 kWh; a 2 kW heater reaches the same 1 kWh in half an hour.
How do I work out an appliance's running cost?＋
Multiply its power in kilowatts by the hours used to get kWh, then multiply by your unit rate. A 2 kW heater for three hours is 6 kWh, which at 25p a unit is 1.50 pounds.
What is the difference between peak and continuous power?＋
Peak is the short burst a device can deliver or draw, often for a few seconds. Continuous is what it can sustain indefinitely. Motors and power supplies are usually specified with both, and only the continuous figure should be used for sizing.
What does power factor mean?＋
In AC circuits, it is the ratio of real power in watts to apparent power in volt-amperes. A poor power factor means more current flows than the useful power alone would suggest, which matters for cable and supply sizing.
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Assumptions & limitations
Engineering formulas are exact; the situations they model are not. Read your result with these limits in mind:
- Gives average power over the period entered, not peak or instantaneous power.
- Assumes 100% efficiency. A real motor draws more input power than it delivers as mechanical output — divide by the efficiency figure on its plate.
- One mechanical horsepower is 745.7 W; metric horsepower (PS) is 735.5 W. Manufacturer figures may use either.
- For rotating machinery, power is torque × angular velocity — use the torque calculator first if you only have a torque figure.