IIRC it is three KW to pump up, and two KW generation on the way down. Therefore the cost per KW nightime had to be 2/3 or less the cost per KW daytime.
The math worked in 1975, not sure now with fairly efficient gas fired combined cycle generation widely installed.
The math still works because of the differential in demand from daytime to nighttime. There is excess capacity at night. Every bit of that capacity costs money to obtain, so instead of wasting it they use it to pump the water, thereby increasing the capacity for daytime use. Win-win.
Don't you stock up on canned goods when they are on sale?