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If the main inverter is alive 24/7, the backup inverter could be a type with inbuilt transfer switch (i.e. UPS functionality) that will detect the loss of the main inverter's output. Or you can get standalone transfer switches that would just switch the freezer from main to standby. But in both cases the standby inverter would also run 24/7. Better would be to add an inhibit relay that shuts down the standby inverter when the main one is alive, using its control input or switching the DC supply.
One thing to watch out for: The starting load of a freezer compressor is very high compared to its running load. It may only use say 150W for ten minutes an hour, but could require a 1500W inverter to reliably start it. Another thing you might want to build in is a delay on the transfer of say 5 minutes. If the main inverter fails while the compressor is running, it will transfer and try to restart against head pressure and usually stall. Normally this will trip the protector in the compressor which will try again after a few minutes. But if instead it trips the standby inverter to overload lockout, it might not then restart at all. This won't be needed if the standby inverter is large enough to sustain the compressor stall current until the protector trips.
One thing to watch out for: The starting load of a freezer compressor is very high compared to its running load. It may only use say 150W for ten minutes an hour, but could require a 1500W inverter to reliably start it. Another thing you might want to build in is a delay on the transfer of say 5 minutes. If the main inverter fails while the compressor is running, it will transfer and try to restart against head pressure and usually stall. Normally this will trip the protector in the compressor which will try again after a few minutes. But if instead it trips the standby inverter to overload lockout, it might not then restart at all. This won't be needed if the standby inverter is large enough to sustain the compressor stall current until the protector trips.
Failure of the cheap Chinese inverter, which I got for free so can't complain.
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