I was having this discussion in a different thread. @wjgjr mentioned that the PW shutdown at 10% when power grid was out. I was surprised, because when I did the Self Powered test with 0% reserve in app (5% GW), the batteries were drained all the way down to 0% in app (5% GW). That's troubling, because in that thread, I was only able to get 11.1kWh per Powerwall when it went all the way down to 0% app (5% GW). So this means I would only have 10.5kWh per Powerwall if there was an actual outage. How come Tesla advertises that PW has 13.5kWh usable energy? The inverter efficiency is 10% round trip, so it can't be due to inverter loss.
Tesla definitely should be more clear on this. As you say, Tesla advertises 13.5 kWh of usable energy (and also 14 kWh total energy, implying they have accounted for reserves and/or losses,) so as a consumer looking at PWs for the first time, I would expect a full discharge to be 13.5 kWh of power to the home, after accounting for any reserves/losses. And, they should advise consumers on what to expect in low battery situations - particularly, if it really will shut off at 5% on the app, they should be clear on that (and this could even be an optional alert notification in the app when you hit 10% plus a warning, similar to when a car hits a low fuel state.) It was certainly a surprise for us in testing.