Abstract: A measurement system and method of forecasting time-dependent corrections into a power output of photovoltaic power generators based on a determination of time-dependent shading of photovoltaic cells. Identification of cloud positioning in the sky is based on recordation of images of a scene within a field-of-view FOV that optionally subtends the Sun, base on which images a velocity vector associated with cloud movement is computed to form output associated with time when clouds will shade power generators in question. A method for producing a weather forecast based on such corrections.