An uninterruptible power supply (UPS) system includes an AC output, an inverter coupled to the AC output and configured to provide power at the AC output, and a switch configured to selectively couple a generator (e.g., an engine-generator set) to the AC output. The system further includes a control circuit configured to variably modulate the switch to gradually increase control power flow from the generator to the AC output while causing the inverter to concurrently provide power to the AC output. The switch may be a static switch that includes at least one silicon controlled rectifier (SCR), and the control circuit may be configured to control a conduction interval of the at least one SCR to control power flow from the generator to the AC output.