This paper presents a novel technique to estimate the wave excitation force which is an essential signal in the control of a Point Absorber Wave Energy Converter (PAWEC). The work uses a nonlinear PAWEC simulation together with a modified form of the well-known fast adaptive actuator fault estimation (FAFE) technique for nonlinear Lipschitz system, the fast adaptive unknown input estimation (FAUIE). The estimated wave excitation force is an important reference input for optimum power control and is considered as an unknown input. The results show accurate wave excitation force estimation based on irregular wave generation as well as the performance of the power tuning controller.