In choosing or designing a machine to do a particular job, important factors to consider include the power input and the power output. You would expect a hairdryer with a high power output to dry your hair faster than one with a low power output, but a vacuum cleaner with a high power input may not clean the carpet any faster than one with a lower power input, it may just be less efficient!

Do not confuse power with speed. A doubledecker bus may be slower than many cars, but it can transport sixty passengers a given distance much faster than any car!

Steam trains have a very high power input, but a very low efficiency at transferring this to output power.

Power is a measure of the work done or energy transferred each second.

Power is the rate of working or energy transfer.

Average power can be calculated as work done ÷ time taken, P = ΔW÷Δt
Power is measured in watts (W), where 1 watt is a rate of working of 1 joule per second (J s–1)

Typically, a car engine has an output power of 70 kW, compared to 1.5 kW for a hairdryer and 2 W for a clock.