I'm not sure that I'm following what you're trying to say here. It's not like there is 'excess' power that's being dumped (to ground, for example).
The generation system has a feedback component that causes the generators themselves to control generation such that the amount being used matches the amount being generated (leaving aside inevitable resistive losses).
This feedback component covers normal variations in load. For more substantial changes in load, peaker plants can be ramped up as needed to cover the shortfall. Hydro plants make the best peakers as they don't need to generate steam prior to generating power.
It will, however, cause the generators to generate less electricity which would require less water (Hydro) or burn less fossil fuel (NG/Coal).
There are several million households in the USA. A couple of watts saved at each one adds up to two or three full-sized power-plants.
Sure there is. Water isn't limitless. Once there is no more in storage, there is no more power. There are also other constraints on stored-water generation systems (i.e. releases during fish hatching season or for irrigation purposes).
Consider a pure storage system such as the San Luis Reservoir - water is pumped into the reservoir during the rainy season (at a cost) and released during the dry season (generating power during release).