by Norman Rogers
The main problem with either wind or solar is that they generate electricity erratically, depending on the wind or sunshine. In contrast, a fossil-fuel plant can generate electricity predictably upon request. Blackouts are very expensive for society, so grid operators and designers go to a lot of trouble to make sure that blackouts are rare. The electrical grid should have spare capacity sufficient to meet the largest demand peaks even when some plants are out of commission. Plants in spinning reserve status stand by ready to take over if a plant trips (breaks down). Injecting erratic electricity into the grid means that other plants have to seesaw output to balance the ups and downs of wind or solar.
Adding wind or solar to a grid does not mean that existing fossil fuel plants can be retired. Often, neither wind nor solar is working and at those times a full complement of fossil fuel plants, or sometimes nuclear or hydro plants, must be available. Both wind and solar have pronounced seasonality. During low output times, as for summer wind, the fossil-fuel plants are carrying more of the load. Of course, solar stops working as the sun sets.