The upper reservoir is a battery. It can be charged by pumping...

The upper reservoir is a battery. It can be charged by pumping water up the hill using the power generated from their own panels (e.g. during the middle of the day when the grid does not require a lot of power because of all of the solar panels on domestic roofs in north QLD), or using power from the grid in the middle of the night when power requirements are low and power is therefore cheap (remember off-peak tariffs?).

Modern pumped hydro plants are around 80% round trip efficient, not far off modern Li batteries, except that the pumped hydro will work without loss of capacity for more than 50 years. Yes, the system is designed/intended to produce power for a relatively short period of time - typically the morning and evening peak power use periods. This link has some more information on the "duck curve" that you might find interesting.