This paper presents a detailed assessment of the spectral accuracy of state of the art numerical estimations of wave energy at the SEMREV French Atlantic test site, by comparison with in-situ measurements. Power density frequency spectra from HOMERE Boudiere et al. (2013), the most up-to-date highly refined hindcast dataset, are compared with several measurements from three wave buoys located either offshore or onsite. The spectral signature of the error exhibits a highly non-linear site dependent behavior. Examined in conjunction with usual comparisons of standard integral parameters, this provides meaningful insight into the epistemic uncertainties and errors in different part of the wave energy spectrum. Notably, a complementary analysis of the mean available energy as a function of frequency illustrates the varying degree of impact that inaccuracies in the estimation could have on production by WECs, which are often designed to harvest energy from specific frequency bandwidths. It notably also enlighten the frequency domains where the input forcing and the accounting or parameterization of processes may still lack accuracy. (C) 2017 Elsevier Ltd. All rights reserved.