How to Calculate Power in Watts from Work and Time

Calculates power as work ÷ time, the rate of doing work. Work is in joules, time in seconds, and power in watts (W).

Explanation

Power is the rate of doing work: how much work gets done per second. The same job finished in less time means more power.

P = \dfrac{W}{t}

$P$ is the power, in watts (W)
$W$ is the work done, in joules (J)
$t$ is the time taken, in seconds (s)

One watt is one joule per second, so 1 W = 1 J/s.

Alongside watts, the calculator gives the answer in metric horsepower (PS), taking 1 PS = 735.5 W. That is the horsepower quoted for cars in Japan and much of Europe; imperial horsepower (hp) is a slightly different unit, about 745.7 W.

Example

The defaults are 600 J of work done in 5 s.

P = \dfrac{600}{5} = 120\,\mathrm{W}

The power is 120 W, which is 120 ÷ 735.5 ≈ 0.163 PS.

Notes

What you get is the average power over the interval. If the effort sped up and slowed down along the way, the average hides that.

The time must be greater than zero. Zero or a negative value gives an error.

When a constant force $F$ pushes something along at a steady speed $v$ , the power is also $P = F v$ . A 60 kg person climbing 3 m of stairs in 5 seconds does 60 × 9.8 × 3 = 1764 J of work, an output of 1764 ÷ 5 ≈ 353 W.