Not only that, there is forecasting of the energy profile to use for each day and what unit and unit fuel source will be used based on the energy load demand. Even without renewables, you need to forecast what units to use because of units output restrictions, fuel cost and load demand.
Not every unit is available all the time and that is why you need to do farecasting for the days ahead. Renewables are not the only energy source that is not available at certain times, it happens to gas, petroleum and every fossil fuel unit. What they are trying to sell is very far from reality!