This contribution presents the control of the electrical system of a Wave Energy Converter (WEC) prototype developed by SINN Power. Due to the movement of the waves, the generated power has a very high fluctuation with a period of a few seconds. To be able to use this power, it has to be smoothed. The used Energy Storage System (ESS) is a supercapacitor bank, which is directly connected to the DC-link. Therefore, the DC-link voltage has to fluctuate according to the generated power, to charge and discharge the capacitors. The smoothed power is used to charge batteries with a DC/DC converter, which is typically used for photovoltaic applications. The DC-link voltage can be controlled with the current through the DC/DC converter, yielding a nonlinear control system where a stability analysis is carried out to prove a safe and stable operation. Measurement results at the prototype under typical sea conditions are presented, which fit the simulation results. With the presented control system, smooth power output can be guaranteed.