Power is defined as the time rate at which work is done or energy is transferred. The average power of a force is defined as the ratio of the work, W, to the total time t taken

`P_(av) = W/t`

The instantaneous power is defined as the limiting value of the average power as time interval approaches zero.

`P = (dW)/(dt)`

The work dW done by a force F for a displacement dr is `dW = F.dr` The instantaneous power can also be expressed as

`P = F.(dr)/(dt) = F.v`

where v is the instantaneous velocity when the force is F.

Power, like work and energy, is a scalar quantity. Its dimensions are `[ML^2T^(-3) ].` In the SI, its unit is called a watt (W). The watt is `1 J s^(-1).` The unit of power is named after James Watt, one of the innovators of the steam engine in the
eighteenth century. There is another unit of power, namely the horse-power (hp)

`1 hp = 746 W`

This unit is still used to describe the output of automobiles, motorbikes, etc.
We encounter the unit watt when we buy electrical goods such as bulbs, heaters and refrigerators. A 100 watt bulb which is on for 10
hours uses 1 kilowatt hour (kWh) of energy.

= 100 (watt) x 10 (hour)

= 1000 watt hour

=1 kilowatt hour (kWh)

= `10^3 (W) xx 3600 (s)`

`= 3.6 xx 10^6 J`