The formula
P = W / t
| Symbol | Quantity | Unit |
|---|---|---|
| P | Power | W |
| W | Work | J |
| t | Time | s |
What it means
Power measures the rate of doing work. Climbing a flight of stairs takes the same energy whether you walk or run, but running takes less time and therefore more power.
How to work it out by hand
- Work out the work done in joules
- Divide by the time taken in seconds
- The answer is in watts
What is worth knowing
The stairs example is the clearest illustration and worth carrying: walking and running use the same energy against gravity, but the power differs by the ratio of the times. James Watt coined horsepower to sell steam engines against the animals they replaced, defining it as roughly what a horse could sustain — 746 watts, which is generous to the horse. A fit human sustains about 100 W over long periods and can peak above 1000 W briefly, which is why bicycle power meters read the numbers they do.