This paper proposes an autonomous control for parallel-connected uninterruptible power supply (UPS) systems based on electrical power system interrelations. The controllable variables are the output voltage phase angle and magnitude. Relation between active current and output voltage phase angle is derived from a synchronous machine model. Electrical power systems suggest that voltage regulation and reactive current can be proportionally related to yield another control equation. The resulted control scheme is very simple and only requires an output current measurement to provide a voltage reference signal locally in synchronism with other UPSs without communication signal exchange. A small-signal model represents the entire system and constitutes proof of its stability. Feasibility is confirmed by an experimental setup with three UPSs connected in parallel with different output power ratings. In the experiments, voltage limiters (1%) are employed and three cases are verified. Under all conditions, the results show excellent performance in terms of stability, and active and reactive load sharing.