An adapter is used to couple a prime mover body to a housing of a generator. The adapter has openings that allow an armature of the generator to be attached to the flywheel of the prime mover. The adapter provides a torque coupling between the prime mover and the generator so that the attachments of the prime mover and generator to the skid, as well as the skid itself, can be reduced in size and weight because they no longer need to oppose the prime mover-generator torque load. A thickness of the adapter and a pattern of the openings in the body of the adapter are selected to move any natural resonance frequencies of the adapter outside an operating frequency of the prime mover.