Power measures the rate at which work is done. The SI unit of power is watt, and it is defined as one joule of work done in one second. Recall that work is the transfer of energy, therefore, power can be represented as the rate of transfer of energy. For example, a 100 watts light bulb shines brighter than a 50 watts light bulb because the energy emitted per second by the 100 watts bulb is double that of the 50 watts bulb. Similarly, two weightlifters will do the same amount of work while lifting a weight to a specific height, but the one doing it in a shorter time will have more power output. When the work done varies as a function of time, the power obtained by dividing the total work done by the total time taken is called the average power. If the time intervals are reduced to find the power output at a particular point, it is called instantaneous power. It is equal to the average power when the power is constant over a time interval.