Volatility and unpredictability of sources
Renewable energy sources are by their nature volatile and unpredictable.
For example, it is not possible to predict with complete certainty the intensity of a wind or solar radiation in the short term or whether enough rain will fall to make river flows sufficient for hydroelectric production or the like.
The volatile nature of renewable energy sources limits the possibilities of their feeding the grid. Electricity production from renewable energy sources varies greatly during the day making it hard to plan when the power plant will be in operation (and at what capacity) or out of operation.
Accordingly, there has to be a sufficient reserve in the system of instantly available installed capacity capable of covering for the shortage caused by a renewable source going out of operation (e.g. a wind power plant cannot operate when there is no wind).
Further, the power grid in a particular location can only receive a certain amount of electricity without risking being overloaded and jeopardizing the system stability.
The greatest difficulties in feeding the grid occur with wind power plants, which is therefore limited to the power acceptable for the grid and for the secure and stable operation of the whole system.