So let's say a power plant is powering a city. At one point in the day the city is using some amount of power, lets say 100 GWatts. At another point in the day the city might be using 10 Gwatts of power. How does the power plant know the difference? I mean, how is the difference in the magnitude of the loads and the power being consumed perceived by the generator? It has to spin at the same speed no matter what, is there a difference in energy required to spin the turbines due to less magnetic opposition when there is a lighter load or what? I've found a lot on power transmission, transformers, and phasing on the internet, but none of the sources I've found really touch on the exact conversions of energy that happen during the power cycle. If hardly any power is being required by the city, doesn't the power plant still have to convert the same amount of kinetic energy to electrical energy to spin the turbines? I'm confused. Also, this might be an unrelated question, but how do power plants receive power? I've heard about people decking out their houses with solar equipment and actually getting a paycheck from the electric company.