how much power does a step down transformer used?

A step-down transformer does not consume power; instead, it transfers electrical power from a higher voltage to a lower voltage. The power input (primary side) is approximately equal to the power output (secondary side), minus any losses due to inefficiencies, typically expressed in watts (W). For example, if a transformer steps down 240V to 120V and has an efficiency of 95%, the output power will be 95% of the input power.