HZ is short for Hertz, which is the unit of an international standard unit of frequency of an alternating current voltage. 50HZ means the voltage alternates 50 times per second and the 60HZ means it alternates 60 times a second. In short, the answer depends on the power supply where the pump is being used, UK and Europe generally have a 50HZ power supply, but Americas and other parts of the world use 60HZ. It is important to match the pump to the power supply to the pump operates at it.