Converting Between Mechanical Power And Electrical Power In the SI system there is no difference between electrical and mechanical quantities. Thus power and energy have the units of watts and joules respectively, regardless of whether we are considering mechanical or electrical quantities. In the imperial system the unit of mechanical power is horse power (hp) and these must be converted to watts when considering electrical power. In either case, when converting from mechanical to electrical power or visa versa, efficiencies must be taken into account.

Example

Calculate the power of a lift which can raise a mass of 800kg through a height of 50m in 98.1s.

work done = force × distance

The motor must overcome the force due to the weight (not mass) of the lift. The weight of the lift equals its mass multiplied by the acceleration due to gravity (9.8m/s2).

Calculate the output and input powers of a motor driving the lift if the lift gearing is 80% efficient and the motor is 90% efficient.

The output from the motor must be enough to provide the lift with enough power once the losses in the gears have been taken into account.

input to lift including gears=output of motor=output from gears=4*100=5kw

The electrical power needed to drive the motor must be enough to provide 5kW of power from the motor after the losses incurred while converting electrical power into mechanical power.