A refrigerant cycle is provided with a control for an expansion device to achieve a desired compressor discharge pressure. The system operates transcritically, such that greater freedom over compressor discharge pressure is provided. The system's efficiency is optimized by selecting an optimum discharge pressure. The optimum discharge pressure is selected based upon sensed environmental conditions, and the expansion device is adjusted to achieve the desired compressor discharge pressure. A feedback loop may be provided to sense the actual compressor discharge pressure and adjust the actual compressor discharge pressure by further refining the expansion device. The system is disclosed providing heated water based upon a demand for a particular hot water temperature. Further, the optimum discharge pressures may be determined experimentally, with an offset added to the experimentally determined value to ensure that the actual pressure is higher than the desired, or optimum pressure for the particular refrigerant cycle. In one embodiment, a formula is utilized to determine the optimum discharge pressure.