109,500 gallons of diesel/year is ~$265,100/year[1].

1.4 Megawatts (5328 solar panels) is from ~$4.3M to ~$7M[2].

60 Tesla Powerpacks (6 Megawatts/hour) is ~$8.7M[3]

So it's at worst a 50 year bet (if gallon's price doesn't increase) which is pretty good for a reliable continuous source of power.

[1]: https://www.wolframalpha.com/input/?i=how+much+is+109,500+ga...

[2]: http://www.freecleansolar.com/What-is-the-price-cost-for-a-S...

[3]: https://en.wikipedia.org/wiki/Tesla_Powerwall

I think it would be interesting to read about the decision process that decided on the 1.4MW of power generation to 6MWh of storage ratio. This is around 1:4.2h. Is this ratio very specific to Ta’u island, with their usage patterns, or can this ratio be used as a guide for future installations? I'm guessing that Ta'u has not as much technology that stays on overnight, and a more realistic ratio would be 1:6h or more.

Anyway, this is a great proof of tech.