Solar inverter with mains backup
If you have a solar powered inverter that works some of the time (because the sun don't shine all the time) AND your inverter works from 12V (from some kind of solar regulator and battery?) then why complicate things by trying to get your fridge/freezer dual powered from the inverter output and mains AC.

Simplest way (to me) is take your AC mains, step it down with a transformer, convert to DC (bridge rectifier), regulate to +13.4 volts and feed onto the 12V your inverter is using. Call it a "power supply".

If the solar stuff aint producing the goods (energy wise), the 12 volts will be topped-up from the power supply. If the solar panels are producing the energy then, by careful control of the "power-supply", it'll barely consume any energy.

I think this overcomes several headaches of trying to ensure your solar inverter AC output doesan' "clash" with the real AC mains.