Power Calculator (P = W/t)Free online science calculator — instant results, no sign-up
Calculate power from work done over time.
About the Power Calculator (P = W/t)
The Power Calculator (P = W/t) calculates average power by dividing work done in joules by elapsed time in seconds. It displays the result in watts, kilowatts, and horsepower. With the default inputs of 5000 J and 10 s, it returns 500 W, 0.5 kW, and 0.6705 horsepower.
- Work (J)
- The work done or energy transferred during the measured interval, expressed in joules (J), with a default of 5000 J.
- Time (s)
- The elapsed time over which the work is done, expressed in seconds (s), with a default of 10 s.
Formula and method
P = W/t; P_kW = P/1000; P_hp = P/745.7. With the defaults: P = 5000/10 = 500 W; P_kW = 500/1000 = 0.5 kW; P_hp = 500/745.7 ≈ 0.6705 hp.
- PAverage power in watts (W), where one watt equals one joule per second.
- WWork done in joules (J); the variable W denotes work, whereas W after a numerical power value denotes the unit watt.
- tElapsed time in seconds (s).
- P_kWThe same average power expressed in kilowatts (kW).
- P_hpThe same average power expressed in mechanical horsepower (hp), using approximately 745.7 watts per horsepower.
Divide the entered work by the positive elapsed time to obtain average power in watts. Divide that result by 1000 for kilowatts and by approximately 745.7 for mechanical horsepower, rounding the horsepower result to four decimal places for the worked example.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Work5000 J
- Time10 s
Results
- Power500 W
- In kilowatts0.5 kW
- In horsepower0.6705
How to use the Power Calculator (P = W/t)
- 1Enter the work done in Work (J); use 5000 for the default example.
- 2Enter the elapsed time in Time (s); use 10 for the default example and ensure the time is greater than zero.
- 3Calculate power by dividing 5000 J by 10 s to obtain 500 W.
- 4Read the converted outputs: 0.5 kW and 0.6705 horsepower.
Reading your result
The Power row shows average energy transfer per second: 500 W means an average of 500 joules per second over the entered interval. The In kilowatts row expresses that same rate as 0.5 kW. The In horsepower row expresses it as 0.6705 mechanical horsepower, rounded to four decimal places. These rows represent one power level in different units, not separate quantities to add together.
Limitations
The result is average power over the entered interval, not instantaneous or peak power unless the rate of work is constant. Time must be greater than zero. Work and time must describe the same process and interval, and inputs in other units must first be converted to joules and seconds. The calculation does not account for efficiency or losses unless those are already reflected in the work value. Horsepower uses the mechanical definition, not metric horsepower, and displayed precision does not improve the accuracy of the inputs.
Common questions
Why do 5000 J and 10 s give 500 W rather than 5000 W?+
Power measures how quickly work is done, not the total work itself. Dividing 5000 joules by 10 seconds gives 500 joules per second, which equals 500 watts.
Does this calculator report peak power?+
No. W/t gives average power across the entered time interval. Finding peak power requires information about how the work rate changes during that interval.
Which definition of horsepower produces 0.6705?+
The conversion uses mechanical horsepower, approximately 745.7 W per hp. Dividing the default result of 500 W by 745.7 gives approximately 0.6705 hp; metric horsepower would give a different value.
Can I enter zero for Time (s)?+
No. Dividing work by zero is undefined, so zero elapsed time cannot produce a valid power result. Enter a positive duration corresponding to the work measured.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
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Results are estimates for informational purposes and are not financial, medical or legal advice.