We consider the problem of synchronizing two electric power generators, one of which (the leader) is serving a time-varying load, so that they can ultimately be connected to form a single power system. Both generators are described by second-order reduced state-space models, and we assume that the generator not serving any load initially (the follower) has access to measurements of the leader generator phase angle corrupted by some additive disturbances. By using these measurements, and leveraging results on reduced-order observers with ISS-type robustness, we propose a procedure that drives (i) the angular velocity of the follower close enough to that of the leader, and (ii) the phase angle of the follower close enough to that of the point at which both systems will be electrically connected. An explicit bound on the synchronization error in terms of the measurement disturbance and the variations in the electrical load served by the leader is computed. We illustrate the procedure via numerical simulations.