PURPOSE: To provide a power saving unit in which the output voltage can be switched smoothly depending on the supply voltage without effecting on the operating electric machines and appliances.

CONSTITUTION: In a single phase three-wire power saving unit, a voltage detector 1 measures the voltage between terminals (u) and (o) and delivers a signal when a voltage higher than a set level is detected. Consequently, a contact 12 is opened to turn a relay coil 40 OFF thus opening the contacts 42, 43. At the same time, a relay coil 10 is conducted to close a contact 11 and a relay coil 30 is conducted to close the contacts 32, 33 thus causing current flow between the terminals 2a, 2c and 3a, 3c of transformers 2, 3. Consequently, a voltage corresponding to the turn ratio appears between the terminals 2b, 3b thus causing output voltage drop and thereby power saving. When the output voltage drops below a set level, each part of power saving unit is reset and the output voltage is substantially equalized to the supply voltage.