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...

Follow the example. It doesn't require 100% efficiency. You have a
river
that's flowing at 100MW an hour. During the day, there is a market
for
all that power. At night the demand is only 60MW. So, you could let
that water pass by unused or use the 40MW available to pump water
to a reservoir above the power plant. Then, during the day when you
need more than 100MW, you release some of that water. You now
have more than 100MW of water availble to turn the generator
because you have EXTRA water beyond what the river supplies
during that peak period.
Hence you can produce more than 100MW of power during the
day when you need it.



That makes no sense at all. You are saying let the water flow past the dam.
Then pump it back up to the top ? It is not like getting the water down
stream 10 miles or more from the dam. Why not just cut the flow back to the
demand for power.