It is important to understand the relationship between the ambient ebb and flood currents and the electricity generated by tidal stream power generators to minimise investment risk and to optimise power generation for distribution purposes. Such analyses no longer rely on average descriptions of the flow field or on single values for the device efficiency. In the present paper, we demonstrate a new method involving the integration of synthesised long termflow vectors with logistic descriptions of the device power curves. New experiments are then described with the Neptune Proteus vertical axis tidal stream power generator involving tow tests at speeds to 1.5 ms–1 in William Wright Dock on the Humber. The results are used to derive appropriate coefficients in the logisticcurve and to estimate the device’s annual electrical output.