This article presents an assessment of the optimum mean power output of a point-absorber wave energy converter (PAWEC) in irregular wave climates. A first-order method is used to calculate the mean power spectrum of the device in waves of dissimilar spectra. JONSWAP spectra with different peak enhancement parameters are used to show the effect of energy concentration within the wave spectrum on the device mean power spectrum. A comparison of the optimum mean power output of a PAWEC in an irregular wave climate with that in an energy-equivalent regular wave climate is made to show the influence of the combination of wave spectrum and power output characteristics on the former. The principle is demonstrated by results obtained with the simulation of a PAWEC collector. An evaluation is made of the difference between the optimum mean power output of the device and the mean power output achieved using a simple, standard control method. The resulting difference in energy output with measured data from a possible site is also assessed. The implications of the results on the appraisal of device performance are considered.