Olly,
About 2 months ago, I noticed my inverter switching over to battery at around 2-3am every night and would stay until morning around 7-8am. I have a Technomaster (same as powertek) inverter model TM-1500, its a 12V 1.5KW modified sine wave. This inverter works just like an ups, even gives you the same audible beeps for a few secs when inverter kicks in (power loss/low battery/overvoltage).
At the time I didn't understand why my inverter was kicking in almost at the same time at night, without power loss. After a couple of days I borrowed a voltage meter, and measured the voltage on both phases, and during the day it was at 131.5 volts and it would creep up at night to 135.0-135.9, and both phases together was 270 volts! I was shocked, and now knew why my inverter was doing what is was, it has a fixed overvolt protection set at 135volts, so it switches over to inverter mode to protect anything plugged after it, and will not switch over until the voltage is dropped below 135volts.
I live in an apt complex so I measured the voltage in the common area, just to make sure there is nothing going on with my circuit, and it was the same voltage as well.
I called Edeeste and reported the problem around 2am, and they came out around 8am that morning. They measured the voltage at the main panel for the building at was same results as mine single phase 132+ and 265+ both phases. They fixed the problem by adjusting the transformer we have for the building, and now its 120 and 240.
I'm still amazed that my A/C, fridge, or microwave didn't burn out due to the over voltage since they are not protected by the inverter.
Thanks.