I tested this today. At 97% off grid I was still seeing 59.9 Hz. At low 98%, it rose to 60.2. Midway through 98% it hit 60.5 and the inverter shut off. At this point I did a full Insteon device status query. Two devices failed to respond (including retries). The fact that most of the devices still talk ok is almost certainly because most of them are dual mode Insteon and also use wireless as a redundancy. So all in all even if I'm off grid with a full charge, not a big deal.