Timing determination for UPS power transfer

Disclosed are devices, systems, and methods for operating a backup power source or an uninterruptible power supply (UPS) that can be used in data centers and that provide a backup power source to power the data center when utility power is compromised. A power delivery system that provides power to a primary system may include a UPS with a state timing control system that operates a bypass static switch. The state timing control system can determine when to transition the primary system from the utility power supply to the backup UPS, based on the current AC voltage conditions. The state timing control system may perform modeling to emulate the intermediate DC voltage of an actual rectifier, and particularly emulate the holdup capacitor voltage. The emulated capacitor voltage can be obtained in real time by both an input power model based on RMS utility voltage and the actual rectifier output load.