System Efficiency:
Assume that we connect the apparatus of figure as shown below and do the experiment explained. The wattmeter may show something such as 800 W. In this situation we can compute the efficiency of the entire system by dividing the actual mechanical power (700 W) by the measured input power (800 W). When we call the input power Pin and the actual mechanical power Pout, then the effectiveness Eff is given by
Eff = P out/Pin
When you want to compute the efficiency in percent, Eff%, use this formula
Eff% = 100P out/Pin
Figure: Electrical power can be measured directly whenever a motor is used to drive a winch to lift a heavy object.