Gravity potential energy = mgh. Suppose pool is 10 meters up, then you need 239,000 kg of water = 63K gallons or 8400 cu ft. This is a huge pool: 29 ft. x 29 ft x 10 ft.
You could use the city water supply: borrow water through a turbine at night, and return it during the day :-)
Or have pool up very high, for example on a cliff overlooking the Mediterranean:
Could I efficiently use excess solar to pump water in to a house roof storage area and generate electric from that?