We consider the problem to synchronize two electric power generators, one of which – the so-called leader – serves a time-varying load, so that the two can be connected to ultimately form a single power system. Each generator is represented by a second-order reduced state-space model. We assume that the generator that serves no load initially – the so-called follower – has access to measurements of the leader generator phase angle corrupted by some additive disturbances. We deploy these measurements and leverage results on reduced-order observers with ISS-type robustness to 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 can 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.