An exemplary embodiment of a system includes first and second sensors. The first sensor is operable to provide an output indicative of a sensed refrigerant liquid temperature of a liquid refrigerant line that is within or extending from an outlet of a condenser coil of an air conditioner or heat pump unit. The second sensor is operable to provide an output indicative of a sensed refrigerant liquid pressure in the liquid refrigerant line. A controller is configured to determine at least one target pressure value from the output indicative of the sensed refrigerant liquid temperature of the liquid refrigerant line. The controller is configured to determine if the level of refrigerant charge is at, above, or below an acceptable level based on a comparison of the output indicative of sensed refrigerant liquid pressure to the at least one target pressure value.