Predicting electrochemical performance directly from first principles remains a major challenge, as conventional approaches fail to capture dynamic surface coverage and constant-potential conditions at electrified interfaces. Here we develop an operando simulation framework that enables constant-potential, multi-scale modelling of electrochemical reactions with explicit solvent and evolving surface charge. Using acidic hydrogen evolution on Pt(111) as a model system, we resolve how surface hydrogen coverage governs interfacial structure, reaction energetics, and elementary kinetics across Volmer, Heyrovsky and Tafel steps. Building on these insights, we construct a fully coupled microkinetic description that links electrode potential, pH, and surface hydrogen coverage. This enables, for the first time, the direct simulation of current-potential polarisation curves and Tafel behaviour from first principles, in quantitative agreement with experiments. Our results reveal that hydrogen evolution activity is fundamentally limited by a maximum achievable surface hydrogen coverage, arising from the competition between hydrogen formation and consumption steps. This establishes surface coverage as a key design parameter and provides a new physical origin for limiting current behaviour beyond proton transport constraints. This framework offers a general route to predict electrochemical performance from atomic-scale simulations directly.