There is zero chance right now of any battery storage being in the $3/kwh range. Even in the manager-of-the-battery-factory-took-some-home-for-cost situation.
A Tesla battery is roughly 80kWh, and costs $5k - $10k. If it were $3/kWh then it would cost $240. And since most home batteries are 10kWh they would cost $30 (plus electrician labor costs and wiring).
That would be one of these [https://www.amazon.com/LiFePO4-Battery-Perfect-Applications-...] for $3, if we were only talking batteries. No MPPT or line chargers, etc.
That’s about what the plastic and internal bus components are going to cost for that battery, with no actual battery internals.