Coal is a baseload plant and doesn't ramp up or down in power, most down ramp down at all, even at night. In the Twin Cities, MN area they have converted a coal plant to natural gas in response to the wind farms, natural gas can rapidly change output in relative to coal which cannot. With solar power grid-tie if there was a cloud of overcast or a sunny day followed by a storm cloud, the power generated would swing heavily and there would need to be plants that could ramp up quickly to handle the rapid change in demand.
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