Pretty sure coal plants are already used in that way, but on a 'grid-wide' level; based on predictions based on the time of day, time of year, and other factors, traditional power plants get scaled up / down or powered up / down, preferably with minimal waste. Wasting fuel and powering up/down isn't profitable for the energy companies either. I can imagine that the fast-responding power plants are the hydroelectric ones, since their power output can be adjusted by just closing off some valves. Of course, if their reservoir is empty or low they can't scale up easily.