The present study investigates the uncertainties in wave power characterisation associated with the computational-methods accuracy of two state of the art phase-averaged numerical models: SWAN and TOMAWAC. The target area is the Sea of Iroise (western Brittany) considered as one of the most energetic regions along the French coasts with a mean offshore energy flux around . Model predictions based on the same settings are first evaluated against available measurements of significant wave height and peak period at nine wave buoys. Wave power predictions are then compared during a eight-year period (2004–2011). Patterns of wave energy flux present in both cases the same spatial and seasonal distributions identifying coastal hot spots off the isles of Ushant and Sein, the bay of Audierne, the headland of Penmarc׳h and the Crozon peninsula. In offshore waters, SWAN provides however wave power estimates by 15% lower than TOMAWAC predictions. A part of these differences is attributed to the computational methods for extracting the wave energy flux from model predictions as SWAN method neglects additional terms in the output evaluation of the total wave potential. Differences are reduced to 5–10% in shallow waters where uncertainties in wave power characterisation appear mainly associated with the inter-annual and inter-seasonal variabilities of the wave climate.