I think a bit more than that, the article says 250 million gallons which is more like 960 million litres. That squares with the dimensions given: 15m x 15m x pi x 75m x 20 tanks = 1 million cubic metres = 1 billion litres.

Oahu uses about 225MW on average in residential electricity [0], so this could store an hour or so of excess capacity. That's not nothing, but you might want to 5x or 10x it to really smooth out demand and supply peaks for the island.

Note that you also need a set of tanks or a lake at the bottom of the hill. The tanks are 75m tall, so how are you accounting for that in your 120m of vertical distance?

[0] https://www.eia.gov/todayinenergy/detail.php?id=49036 gives 2,018 kWh per person in Hawaii

I subtracted 100 from their position on red hill at 220-ish meters to account for the fact that the tanks are buried in the ground (I don't know precisely where).

>Oahu uses about 225MW on average in residential electricity [0], so this could store an hour or so of excess capacity. That's not nothing, but you might want to 5x or 10x it to really smooth out demand and supply peaks for the island.

https://spectrumlocalnews.com/hi/hawaii/top-stories/2022/08/...

This solar plant & battery system was completed last month. The battery is able to store 144 MWh. This shows me that planners and people thinking about this are willing to invest & commit to power solutions at this scale. I don't know if you are aware or have read up on the red hill controversy. These tanks were used to store fuel by the Navy. They leaked earlier this year and poisoned the Red Hill aquifer and have caused quite a bit of local controversy.

I'm struggling with the idea of writing a letter to the editor outlining this concept, but also emphasizing how converting these tanks to a water storage/ energy storage facility would improve local resilience to drought, bring down water costs, improve environmental conditions etc.