The two hydrodynamic coefficients, the damping coefficient and the added-mass coefficient are incorporated into a single complex coefficient, the radiation impedance. These coefficients become matrices for a system of interacting wave-generating oscillators. A derivation and an optimization of the power absorbed by the system are obtained by using a phenomenological theory. Subsequently the phenomenological parameters are related with hydrodynamics. Finally the optimum absorption by two heaving point oscillators is considered. It is demonstrated that the optimum velocity for maximum power absorption is not in all cases in phase with the excitation force.