Grabbing the back of napkin...
In October, wholesale solar started selling the brand new 14KWH powerwall2 for $5,550. http://www.wholesalesolar.com/... $5550/14KWH = $396/KWH
According to https://en.wikipedia.org/wiki/... The Powerwall has a lifespan of 1000-1500 cycles.
So, assuming 1500, that makes the amortized raw cost of storage = $396/1500 = 26 cents per KWH.
The going rate for uninstalled solar panels is about $2/peak watt. If we figure 80% efficiency, 6 peak equiv hours/day in sunny Samoa, 25 year lifespan, then that watt of solar cells nets just under 44KWH. So $2/44 = adds another 4.5 cents per KWH raw cost for the panels (I'm not accounting for the planned 20% degradation over those 25 years)
So, that's about 30.5 cents/KWH for the raw equipment, not including wiring or racking. It also does not include the electronics needed to get DC panel power to the AC powerwall. (or vice-versa, the cost of an inverter if it is DC all the way) Also not including installation or maintenance costs, wiping off bird poop, or the cost of keeping the backup generators in good working order. It also does not include the cost of mishaps (hurricane, sand damage, broken wires, etc). I lost two panels in a recent hail storm, but those were old panels. The newer generation of panels are built to handle baseball-sized hail.
Installed cost per watt for solar adds about 50%. So, this brings it up to about 45 cents per KWH (not including maintenance or back-up capability).
So, this might be cost-effective, but only if they are paying a very high price for diesel generated power.