It is generally more energy-efficient to set your AC unit to "auto" rather than "on." When set to "auto," the unit will only run when it needs to cool the space to reach the desired temperature. This means the AC compressor and fan will cycle on and off as needed, resulting in reduced energy consumption. In contrast, leaving the unit in the "on" position will keep the fan running constantly, even when the temperature has already been reached. While this may provide more consistent airflow, it can lead to higher electricity bills and potential overcooling.