Home battery sizing is easy to oversimplify. A better question is which loads need to run, for how long, and how often the battery can recharge.
A clean capacity discussion reduces wrong expectations before the system reaches site.
5kWh, 10kWh and 15kWh are different user promises
A 5kWh unit is often an essential-load backup product. A 10kWh package can cover more daily self-consumption. A 15kWh package starts to feel like a stronger backup system, but only if inverter output and load separation are sensible.
Usable capacity is lower than nameplate capacity
Depth of discharge, battery reserve and inverter losses mean the user should not expect every nameplate kWh to appear at the socket. This needs to be explained early in the sales process.