You can get a unit which will synchronize with mains so there is a seamless change over, all power has to go through the unit, as there has to be a system so with a power failure the power from the inverter can't be back fed to mains supply, this safety system makes the unit rather expensive.
Any freezer or fridge needs to have zero pressure before starting, so when there is a mains failure without synchronized back up, then there needs to be a gap between failure and back-up cutting in, the simple method is a plug and two sockets, so on power failure the system requiring back-up is simply unplugged from mains supply and plugged into back-up supply, the time taken it is hoped this takes long enough for pressure in any refrigeration device to drop.
Although you can use pneumatic timers to give the delay, this does not allow time for the power to stabilise before returning to mains, so simple one plug and two sockets is likely the best option.
The other method is always to run on the inverter, that way there is no loss of power as there is no change over.
The other problem is earthing, where you have a TN supply, you can't use the DNO earth, it may simply not be there, the easy way around is to power each item from its own uninterrupted power supply, and often also the cheapest, when I looked into doing it a large unit to supply multi computers cost more than many units one for each computer, and it also resulted if the unit fails you only lose one persons work. However today easy way is the laptop with battery built in.
With a generator the plug and socket has a big advantage, as if the emergency backed up items are supplied from consumer unit with is normally plugged into a 32A socket supplied from main consumer unit, if you unplug to plug into a generator should that generator be replaced likely new one will also have a 32A outlet, so no need change anything when generator replaced.
