This is a chart from the BNEF report titled: "Batteries two times too costly for California’s grid" published Jan 10, 2017.
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As you can see from the chart, the all-in cost is dramatically different from the battery-pack cost. We have been largely focused on just the pack cost as we can find it on Tesla website. But utilities focus on all-in costs. Unless Tesla/Musk figures out some way to dramatically lower the gap, all bets are off.
Just to be clear, Tesla's website shows pack + inverter + control equipment + cabling and site support hardware. It seems to come out at $425/kWh for 50+ packs.
Like you, I wonder about the installation costs. But from past experience, Bloomberg has always come in higher on Tesla's costs than we assume here to be reality. Perhaps Bloomberg's pricing information is similarly high.