P = W/t

Where :

P = power in watts (W)

W = work done (or energy expended) in joules (J)

t = time in seconds (s)

Power is the energy (or work done) divided by the time that the energy was expended over.

Power is the rate at which work is done (how fast it can be done), or the rate at which energy is transferred to another form. The faster a machine can complete a task the more powerful it is.

Power is measured in joules per second. A joule per second is a watt (W).

We tend to think of things 'having a lot of energy' when they are in effect 'powerful'.... that is because they can work quickly.The higher the wattage of a system the faster it can do work. For example a 100W bulb is brighter than a 40W one because it transfers the electrical energy to light energy at a faster rate.... therefore more photons hit your eye per second and you think of it as brighter.

Another equation for power is:

P = I V

from electricity

I = Q/t

therefore:

P = QV/t

Now, E = QV (energy transfered by the electricity)

so again we have that power = rate of energy transfer

Useful link

Physics classroom (most suitable for A level students).