Many sea defense structures need to be adapted to the rising sea water level and changing wave climate due to global warming. The accordingly required investments open perspectives for wave energy converters (WECs) - that are built as part of the sea defense structures - to become economically viable. In this paper the average overtopping discharges q of overtopping wave energy converters built in sea defense structures are studied. Physical model tests with this type of devices have been carried out in a wave flume leading to experimentally determined values for the average overtopping discharge q. These experimental data are compared with predicted average overtopping discharges using existing empirical formulae from literature - derived mainly for sea defense structures. Overtopping WECs have small relative crest freeboard heights and smooth slopes to maximize overtopping, which is contradictive to the basic role of sea defense structures. As a consequence, the experimentally achieved average overtopping discharges are situated in a range that is not well covered by the existing traditional prediction formulae. The presented results for linear-slope overtopping WECs fill the gap between those for smooth dikes and those for plain vertical walls. The overtopping behavior in that particular range is discussed in this paper.