In this paper, the wave energy potential in the western coast of France is assessed, analyzing 58 years of data from numerical models at 10 points located between latitudes 43º30'N and 47ºN. The study focuses on the temporal variability at different scales (monthly, seasonal and inter-annual). The northern part of this stretch is the most energetic (wave powers greater than 22 kW/m), with a decrease of the wave power southwards. The results show that both the wave power resource and the energy output of two wave energy converters (WECs) at the Atlantic coast of France have strong intra-annual and inter-annual variability. From one year to another the wave power may present variations of up to 200%, and the WEC energy output may almost double. These results illustrate that the average mean power alone is not sufficient for adequate wave resource quantification, and it is necessary to consider the intra-annual and inter-annual variability of the wave power and the WEC output when analyzing the potential installation of a wave energy farm in a certain area.