I have two Powerwalls 2. If I am reading correctly, each Powerwall holds 13.5 kWh and allows you to use 12.2 kWh. I set my Powerwall settings to 50% Back-up and 50% Self-Power.
Looking at my Powerwall readout I used only 6.4 kWh and the system read that was 50%. With two Powerwalls...