A refrigeration unit, and method of operating same, having a compressor driven by a prime mover. The prime mover is selectively operable in either a continuous mode which includes heating and cooling cycles, or in start-stop cycling mode which includes cooling and null cycles, to control the temperature of a conditioned space relative to a predetermined set point temperature. In one embodiment, the cycle mode is operable only when the set point temperature is in a predetermined temperature range, the limits of which are selectable. The prime mover is operated in the continuous mode, notwithstanding the selection of the cycle mode, when the set point temperature is not within the temperature range authorized for cycle operation. In other embodiments, the time duration and the upper temperature limit of the null cycle are both selectable, with an automatic override of the selected upper temperature limit being provided when the conditioned space contains a frozen load and the selected upper temperature limit of the null cycle would allow the temperature of the conditioned space to rise above a predetermined value. In still other embodiments, in which the prime move includes an engine and a battery, the minimum battery voltage which will re-start the engine to terminate a null cycle, and the minimum battery charging current which will enable the engine to be stopped to initiate a null cycle, are both selectable.