Abstract Seamounts are critical components of the global ocean energy budget, contributing significantly to turbulent dissipation through their interaction with large‐scale flows. However, most previous numerical investigations used smoothed bathymetry that omits small‐scale topographic variability. We use turbulence‐resolving large‐eddy simulations to investigate how bathymetric roughness affects seamount wake energetics. We compare flows around realistic rough seamounts against versions smoothed over varying horizontal scales, and find that dissipation and mixing rates can differ by an order of magnitude between cases. Importantly, seamounts in the parameter regimes where roughness effects are most pronounced (low slope Burger numbers) are concentrated in the Southern Ocean, coinciding with very fast flows and resulting in leading‐order effects for global dissipation due to seamounts. An implication of our results is that model horizontal spacings of at least order 100 m may be necessary to capture the full energetics in most seamount wakes in the Southern Ocean.