In physics, power is the rate at which work is done, or the rate at which energy is consumed. The higher the rate at which work is done, the faster the work is done, and at the same time the faster the energy is used up. For example, in a one-mile-run exam, the faster you run, the faster you are done with it, however you will feel more exhausted because you have used up your energy in a faster rate. 

     When doing calculation, physics is the amount of work done divided by the time it takes to do it, or the energy expended dividedby the time it takes to expend it. In formula, we have:

                                                                      P=W/t or P=E/t

     Where P is obviously referred to power, W means work , E means energy, and t is time. For units, either power or energy uses the unit joule, and we use seconds as the unit of time, so the unit combined here is something like joule per second? Technically yes, but we give it a special name called watts (w), in memory of James Watt, the man who invented the steam engine, yay!

     When using the unit watt, 1 watt equals to expenditure of 1 joule of energy in 1 second.

     Sometimes watt is just way too small for us to measure power. We never see the unit watt in electricity bill, do we? We use kilowatt (kw) instead. 1 kw equals 1000w. We can then print less zeros in the electricity bills because kilowatt is 1000 times greater than watt.