The FCT80 is a new hydraulic transformer, based on the floating cup principle. The transformer has valve plates with three ports, and is controlled by means of setting the rotational position of these valve plates. Contrary to hydraulic pumps and motors, there are no standardised efficiency definitions for this type of integrated transformers. In this paper, general definitions for efficiency and power loss for hydraulic transformers are proposed, which also take the compressibility of the oil into account. This paper also describes and presents the first test results of the overall efficiency of the FCT80.