IMHO, the main parameter seems to be how many time is needed to fill/empty the dam and to apply a reasonable ratio to ensure the dam is working 95% of the time.

IMHO, the main parameter seems to be how many time is needed to fill/empty the dam and to apply a reasonable ratio to ensure the dam is working 95% of the time. From Wikipedia on Hoover Dam: "Filling of Lake Mead began February 1, 1935, ... ... In the latter half of 1936, water levels in Lake Mead were high enough to permit power generation ..." To empty Hoover Dam once every 60/80 years doesn't sound absurd to me.