Hundreds of tons, thousands of times a second. Not quite. ;) <br><br>When the grid needs power, they don't change the output at nuclear stations. Nuclear power stations are very slow to turn up or down. No where near fast enough to meet public demands. Nuclear power stations typically already run at near maximum output, since that's when they are at their most efficient. Basically when the grid needs power they use pumped storage hydroelectric stations. There is one in North Wales called Dinorwig, or more commonly known as "Electric Mountain" since the powerstation is inside of a mountain. It's right next to the train station for the train that goes up Snowdon. And the public can visit. Basically you have a reservoir at the top of a mountain, and when there is a sudden demand for electricity, they open a big giant plug hole and drain the reservoir, forcing water through the turbines to generate electricity. Then when demand for electricity is low, they buy electricity back from the grid, in order to pump the water back up. Works like a giant gravity powered battery.,(reply>), Sam,  that is not the purpose of pumped storage, they are used as a last resort. As long as all the conventional stations are not at full output, they automatically adjust output to match demand, no manual intervention required. Grid load varies constantly and this variation is taken care of as I explained earlier. Should the power plants be nearing 100% such as gas turbines will be started in case they are required to provide capacity for anticipated needs. Pumped storage is the last resort, a safety capacity option as they are near instant to run.,(reply>), Why don't we just use big masses of steel rotating that are powered with abundant electricity and use them when said is needed. If they are oiled well or suspended in another efficient way, they might be very good at keeping their momentum. And they aren't very expensive, you could make steel containers and fill them with sand or rock. Just the suspension is difficult to build. Maybe some kind of magnets would work. But probably not electric ones...,(reply>), Im not sure how about rest of the world, but in Czech Republic we are already using most of our hydroelectric potential and it makes up for only 4% of electric consumption. I guess that most of developed countries are in similar ssitutation.,(reply>), Are hydroelectric power plants on the rise as a replacement for fossil fuels? If so, that opens another area of sustainability, in terms of our influence on the flow of rivers and surrounding ecologies.,(reply>), the also have Gas turbine open cycle stations on very short run up times 20 mins or so