Or in case of hydro-power plants will it continue to generate 100MW Usually hydroelectric plants are placed near a dam, so that during periods of low demand, t

Or in case of hydro-power plants will it continue to generate 100MW Usually hydroelectric plants are placed near a dam, so that during periods of low demand, the dam can be partially closed to slow energy production, and when demand increases, the dam can be opened again. A closed dam is essentially an energy storage device. So it does make sense to conserve electricity during off-peak hours, so that the power grid can store more energy for peak hours. For natural gas and coal plants, for maximum efficiency, the plants should run at more or less a constant rate. This is similar to a car, which gets maximum MPG during constant-speed driving. Since technology for storing electrical energy isn't advanced enough at this point for mass scale storage (we're there for phones and laptops, almost there for cars, but not houses or cities), there is a lot more waste during off-peak times from these types of plants. There are wacky plans to use plug-in hybrids to store energy overnight when electricity demand is low, and feed it back into the grid during the day. EDIT: here's a good Wikipedia article on it: http://en.wikipedia.org/wiki/Grid_energy_storage http://en.wikipedia.org/wiki/Grid_energy_storage