The present invention relates to a wave-power aggregate and a method of extracting energy from wave motion in a liquid by means of a wave-power aggregate. The wave-power aggregate comprises a container which is situated at least partially in the liquid and also comprises an inflow and an outflow. The container is arranged to, under the influence of said wave motion, to let a first part of the container arrange itself in a first position and a second part of the container in a second position, where the first and second position corresponds to different potential energy states. The method comprises the steps of: alternately supplying the container with at least a first and a second fluid respectively via the inflow, where the density of the fluids differs. at least during an initial stage of the method controlling a flow resistance in at least one of the first and second fluids through the container.