The electricity grid has something called a Capacity Factor. Capacity is the ability to produce power measured in Watts. The Capactiy Factor is the fraction of capacity which is actually producing that power at any instant in time. Power demand and supply must be almost exactly in sync at all times to maintain grid stability. This is achieved by turning on and off coal and natural gas power stations, which can be done relatively quickly, to ramp up and ramp down production as demand fluctuates. Capacity must always greatly exceed typical demand so that unusually high demand can be met. Nuclear, wind, solar and hydroelectric power plants always produce whatever power they can independent of demand. This is for a diverse set of safety and environmental reasons. Only fossil fuel plants can "load follow" to meet changes in demand. Like I stated in my original comment, all load following in the US is done with fossil fuels, and that will be the case for many years or several decades to come. Increases in capacity will mostly come from carbon free sources, but until those sources can load follow, either by dumping unused energy in off-peak hours or by storing that energy until it is needed, all load following will be done with fossil fuels. This means additional demand is going to be met entirely with fossil fuels for a long time to come. I hope that helps you understand the concept of marginal demand better.