RMS of output voltage = RMS of input voltage * magnification =1.41.41.414 *10 =10v.
Output power = square of effective value of output voltage/RL =10 2/8 =12.5W.
Power consumption = power supply voltage * effective value of current =18 *10/8 = 22.5w.
Efficiency = output power/power consumption = 12.5/22.5=0.56.