Small turbines are considered one of the most promising technologies for an effective diffusion of renewable energy sources in new installation contexts with a high degree of integration with human activity (e.g. the urban environment). In these new installations, however, the real working conditions can be far from the nominal ones. In particular, the turbine functioning can be noticeably affected by misalignments between the oncoming flow and the axis of the rotor; differently from horizontal-axis wind turbines, whose performance is decreased by a skew angle, H-Darrieus turbines are thought to take advantage from this condition in some cases. In this study, an improved model for the performance prediction of H-Darrieus rotors under skewed flow was developed. In detail, a theoretical approach based on Momentum Models was properly modified to account for the variations induced by the new direction of the flow which invests the rotor. In particular, the modifications in the aerodynamic characteristics of the airfoils, the swept area and the streamtubes distribution were modeled. The performance predictions of the new model were compared both with experimental data available in the technical literature and with the results of wind tunnel tests purposefully carried out on a full scale model of an H-Darrieus turbine. Notable agreement has been constantly obtained between simulations and experiments.