According to [1] the total price for the project will be up to 7e9 AUD or 5.1e9 USD, so the storage cost will be like 15 USD/kWh. Tesla promises that by 2020 they will have batteries costing bellow 100 USD/kWh. The difference is still big by the factor of 6, but at least it is no longer 2 orders of magnitude.

[1] https://www.theguardian.com/australia-news/2017/dec/21/snowy...

I don't know energy units at all so sorry if this is dumb. But is 2 GW the same as 20 * 100 MW? So $74mm * 20 = 1.48 billion which is less than 2.8 ? E.g. battery is cheaper?

The relevant unit is storage (kWh-- kilowatt hour), not power (MW-- megawatt).

Ah ok. So kWh is like the length of time of that amount of energy delivered. What's the 2 GW then, like max through put at a given moment in time? So the two examples water storage lasts a really long time but a smaller relative rate of discharge versus battery could discharge the amount stored a lot quicker?

kWh is for energy stored in this example, the "size of the tank". kWh is analogous to toe (tonne of oil equivalent) or gallons of gas. kW is for power, that is the flow of energy (energy/s), another non SI unit is horsepower. The SI unit for energy is Joule (J), the SI unit for power is J/s. 1 Watt is 1 Joule per second.

Both are relevant, depending on the purpose.