Power measures how quickly energy is transferred or work is done. It is the rate of energy transfer.

Average power is defined as:

where $\Delta E$ is the energy change over time interval $\Delta t$. Since work $W$ is energy transfer, we can write:

The SI unit of power is the watt (W): $1 \text{ W} = 1 \text{ J/s}$. A device with power $P$ transfers $P$ joules each second.

Consider two processes doing the same work $W$ but over different times $\Delta t_1$ and $\Delta t_2$. If $\Delta t_1 < \Delta t_2$, then $P_1 > P_2$. The faster process requires greater power.

Example: Lifting an object of mass $m$ through height $h$ requires work $W = mgh$. The average power is:

$$P = \frac{mgh}{\Delta t}$$

Lifting the object faster (smaller $\Delta t$) requires more power.

Graphically, if energy versus time is plotted, power is the slope. A steeper slope means greater power—energy changes more rapidly.