Abstract: A standby AC electrical power system connected between a load device and the AC line voltage comprises a voltage responsive control circuit connected between the AC line voltage source and a DC to AC inverter circuit. The control circuit generates a continuously pulsating DC control voltage which has an envelope which responds to and follows but at a different rate the increase and decrease in the envelope of the output of a full wave rectifier fed by the AC line voltage. The control voltage remains above a given triggering level when the average value of the rectified output for the current half cycle involved stays above a given monitored level indicating that the average value of the current half cycle of the AC line voltage is above a given low level and drops to the triggering level when the average value of the current half cycle of the AC line voltage drops to the given low level.