Suppose the weightlifter lifts up a 100 kg weight to a height of 2 m above the ground. The work done on the weight is equal to its gain in potential energy

Is it a large amount of energy? It is not! In fact, this is less than the chemical energy you get from eating 400g of rice. Anyone is able to output this amount of energy, however, only a well-trained weightlifter can lift up the weight. The reason is that the work must be done on the weight in a very short time, may be as short as one or two seconds, otherwise the weightlifter could not sustain his muscles in such a state of high tension. Power measures how fast work is being done, or how fast energy is being transferred or converted:

As energy is measured Joules, the unit of power is Joules per second, or Watts (W). . If the weightlifter can lift the weight in 2 s, the average power delivered to the weight is

This is a high power output, being equal to ten times the power consumed by a 100 W light bulb. In fact, even a well-trained weightlifter cannot sustain this power output for a long time.

Power measures the rate of energy transfer, which must not be confused with the amount of energy transfer. A light bulb labeled 100 W consumes electrical energy at a rate of 100 J per second. But the amount of energy consumed would depend on how long the bulb is lighted up. For example, using the light bulb for 8 hours will consume of electrical energy.