As devices move from full-scale prototype to commercial installations, it is important that developers have detailed knowledge of the tidal energy resource. Therefore, the spatial distribution of the tidal currents over the northwest European shelf seas has been examined to improve understanding of the tidal-stream energy resource. Using a three-dimensional hydrodynamic model (ROMS) at ∼1 km spatial resolution, and applying device characteristics of the Seagen-S turbine, we show that the ratio of the amplitudes of the M2 and S2 tidal currents can lead to significant variability in annual practical power generation – variability that is not accounted for when considering only the mean peak spring tidal velocities, as is generally the case in resource feasibility studies. In addition, we show that diurnal inequalities (governed by K1 and O1 tidal constituents) and tidal asymmetries (governed by the relationship between M2 and its compound tide M4) over the northwest European shelf seas can further affect power generation at potential high-energy sites. Based on these variabilities, the spatial distribution of the tidal-stream ‘capacity factor’ has been calculated. We find that mean peak spring tidal velocities can under-estimate the resource by up to 25%, and that annual practical power generation can vary by ∼15% for regions experiencing similar mean peak spring tidal velocities, due to the influence of other tidal constituents. Therefore, even preliminary resource assessments should be based on annual average power density, rather than peak spring tidal velocity.