Explanation
What this measures
Power is how quickly energy is transferred. One watt is one joule every second, so the same energy delivered faster means more power.
How to enter values
Enter the energy in joules and the time in seconds. A kettle that uses 360,000 J in 3 minutes (180 s) runs at 2,000 W. Energy can be negative if you count energy flowing out; the power then has the same sign.
P = E / t
| P | average power in watts |
|---|---|
| E | energy transferred in joules |
| t | elapsed time in seconds |
1,000 J in 10 s
| Energy | 1000 |
|---|---|
| Time | 10 |
| Result | 100 |
1,000 J ÷ 10 s = 100 W
60 J released over 3 s
| Energy | -60 |
|---|---|
| Time | 3 |
| Result | -20 |
-60 J ÷ 3 s = -20 W
No energy over 5 s
| Energy | 0 |
|---|---|
| Time | 5 |
| Result | 0 |
0 J ÷ 5 s = 0 W
Diagram of this result
The diagram writes the formula with your numbers: energy divided by time equals average power.
A screen-reader sentence states the energy, the time and the resulting average power in watts.
Common mistakes
Energy and power are different: joules say how much, watts say how fast. An average over the whole interval does not show peaks. Kilowatt-hours are energy, not power: 1 kWh is 3,600,000 J.
Limits and assumptions
Inputs use fixed SI units; convert other units before you calculate. This is an idealised textbook relation for consistent measurements, not an engineering design or safety assessment. Values outside the supported decimal range are rejected rather than approximated. Calculations keep up to 30 significant input digits and 50 internal digits; only the displayed answer is rounded.
Sources
OpenStax University Physics Volume 1, mechanics: https://openstax.org/details/books/university-physics-volume-1
How do I use other units?
Convert energy to joules and time to seconds first: 1 kWh is 3,600,000 J and 1 minute is 60 s. Divide the answer by 1,000 for kilowatts.