A powerwall is a collection of interconnected lithium-ion cells with an overall capacity of 13.5kW and comes with a 10 year warranty. It can be used with or without solar. With solar, it charges up in the daytime when the sun shines, or from the grid if production is low.
The powerwall is a smart device, that is, it can detect when it needs to charge, where it can be charged from (grid or solar) and when it needs to switch into supply mode. It can be monitored by mobile phone apps or Wifi, and also has the capability of being controlled manually at a distance by computer or phone.