a method for improving the usability of photovoltaic installations by taking account of shading information of adjacent pv installations for forecasting the power output by a relevant pv installation is provided . in particular , cloud movements and cloud shapes are taken into account . this improves the accuracy of the forecast . here , it is advantageous that short - term forecasts in relation to e . g . the next 15 minutes are possible and a substitute energy source can be activated accordingly , in good time , prior to a dip in the power output by the pv installation . the invention can be used e . g . in the field of renewable energies , pv installations or smart grids .