Since solar and wind power are fluctuating energy sources not all of it goes into the grid when the consumer power demand is lower than that actually produced. This is a well-known drawback of solar and wind energy that everyone talks about. What I never found is an assessment of how much of the entire energy production goes lost (assuming that no kind of energy storage is used)? What is the order of magnitude, say for a big solar power plant supplying a little city/town? 10%, 30%, 50%? Does anyone know? Possibly with some respectable bibliographical source that makes this estimate?