A generator set for a transport refrigeration unit that is operable at a first frequency and a second frequency. The generator set includes a controller, a generator and a prime mover coupled to the generator. The prime mover selectively drives the generator in least at a first non-zero speed and a second non-zero speed. The controller can determine whether a transport refrigeration unit has sufficient capacity to maintain a desired temperature within a transport unit by detecting a generator load, a generator load slope condition, and a speed of the prime mover.