I finally extracted the billing data for part of the summer period from Energy Australia. Though they're required to supply it on request, I eventually had to go to the Energy Ombudsman to get it actioned. Anyway, it was as I suspected when I got my most recent bill - they have not been applying daylight saving time (DST) correctly when calculating time of use bills. As a result, I've been charged at shoulder rate for one hour in the evening, when it should have been off-peak rate. Also, one hour at peak rate when it should have been shoulder rate, but the house is almost always running off the battery at that time anyway. I've put in a request to Energy Australia for them to correct the bill, and dobbed them in the Australian Energy Regulator. Still, other people using their powerwalls in the same way might want to get their bills checked. Note that retailers vary in their approach to this. In their tariff fact sheets some expressly say that they always use standard time. Energy Australia actually say they'll use DST. If your retailer says nothing either way, then the Standard Time Act (NSW) says that DST applies (in NSW), so don't let them wriggle out with "Oh, we meant we use standard time" because it won't wash. Other states with DST probably have similar legislation, but I haven't checked.