What is the term for the rate at which work is done?

Power is the rate at which work is done. In physics, it measures how quickly energy is transferred or converted, rather than how much energy is involved in total.

Average power is calculated as work divided by elapsed time: P = W/t. Instantaneous power is the rate of change of work, written P = dW/dt. The SI unit is the watt, named after Scottish engineer James Watt; one watt equals one joule per second.

Two machines can perform the same amount of work but have different power ratings. A powerful motor completes the task in less time, while a less powerful motor takes longer. This is why a light bulb’s wattage describes its rate of energy use, not the total energy it consumes over its entire lifetime.

In mechanics, power can be written as the dot product P = F · v, linking force with velocity. In electrical circuits, power is commonly P = IV, where I is current and V is voltage. Horsepower is another unit; one mechanical horsepower is approximately 745.7 watts.