the cost to generate power from a battery is still pretty high and if you average in the number of days the pv system does not produce anything or very little, then the cost to generate power from the battery goes up because you may use more than 80% DOD and shorten the lifespan..
But the contract already figures that in.
We don't know yet what actual DOD that 100MWh figure assumed, but
the other Kauai storage system, the 13 MW/52 MWh SolarCity/Tesla system coming on line this quarter,
appears to be spec'd at 100% DOD at the output (so there's probably hidden margin inside, and the price includes cell replacements); see
If the new system is similarly spec'd at 100% of DOD, then that storage cost is getting pretty low indeed.