Just as a point of reference, I have a standard 10si unit, I think about 57 or 60 amp capacity. I take regular road trips, 5-10 hours. With AC, headlights, stereo, electric fuel pump, points ignition, cruise control, electric choke, probably some others I can't remember the alternator keeps up with demand. Don't worry about trailer brake controller, it uses very little power except when you are actually braking. The battery will handle those intermittent loads. If you have an electric fan and electric wipers that may require the next level of output. I have been well satisfied with my alternator.
