Underground pumped hydro storage utilizes abandoned mines as base assets to enhance the grid and add renewable energy. The facilities take advantage of geologic leverage with more energy storage capability while rebuilding retired mining towns that had once prospered.
[pdf] A lithium-ion power inverter is an integrated system combining high-capacity lithium-ion batteries with electronic circuitry to convert DC power to AC electricity (110V/220V). These inverters support solar energy storage, off-grid power, and emergency backup with 85–95% efficiency.
[pdf] $280 - $580 per kWh (installed cost), though of course this will vary from region to region depending on economic levels. For large containerized systems (e.g., 100 kWh or more), the cost can drop to $180 - $300 per kWh.
[pdf] 
