We introduce a moment-based framework to design robust energy-maximising optimal controllers for Wave Energy Converters (WECs). The technique explicitly allows model uncertainty in the computation of the optimal control input, by defining a suitable uncertainty polytope. The resulting robust optimisation is formulated as a minimax problem which has to be solved only at a small number of points of this uncertainty set. The objective function under the proposed strategy is shown to be of quadratic-type and the optimal solution is proven to be unique, providing a computationally efficient robust optimal control framework for WECs. The performance of the proposed controller is demonstrated by means of an application case, which considers a heaving point absorber WEC with imprecisely known model parameters.