Vakie

And of course you must not select a automatic xfer switch, you need to introduce a manual but electrically operated arrangement. Because you need to give a window of time for the smart switches to switch off first, before you command the xfer switch onto the essential output of the inverter. 

Thanks @BritishRacingGreen, I'm going to ask the electrician for an estimate on running a separate cable and compare it to the automation solution. It's not that far, probably 20m from the inverter location and 10m from the main DB. He would likely have to remove some bricks from the paving and I have no idea how difficult / expensive it is, but it sounds like a pain.

A small interruption isn't an issue for my use case, it's much better than a full 2 hours of loadshedding and it's mostly just for running some lights and a tv in the outbuilding which can just start up again without causing any major issues and since I went for a more expensive Deye (I'm glad I did since I was planning on going Luxpower but they didn't have stock) I should be able to still use the excess solar to offset the grid usage of the outbuilding load when the grid is connected even though it's connected to the nonessential load at that time. Now I just need to force the electrician to install the CT...

Any recommendations for alternatives to the Sonoff smart loadshedder or tips / tricks with it? Does it play nicely with Home Assistant?

Hi @Vakie to me the most reliable solution would be to move the outbuilding feed onto the non-essential side, and introduce seperate 2.5mm or 4.00 mm surfix feed  to the outbuilding from the inverter essential side, in order to feed the flat's essentials (lights and plugs). Of course that feasible if the distance between house and flat is not a mile long!
The problem with an aircon and geyser on the essential side is that of racing conditions. Once the grid fails and these loads are on, the inverter will trip in battery mode before you have the time to switch them off via automation techniques. 
EDIT: ok, I get your proposal, you want to keep the flat on  nonessential by default when grid is available, via auto xfer switch. Then when grid fails, you switch the geyser and aircon off via smart switch first , then command the xfer switch to essential. Yes that should work. 
Only small downside is a short interruption on plugs and lights in the flat, but i am sure that may not be a problem for you. 
Good thinking, as long as the cost of automation does not exceed the cost of a seperate 2.5mm cable feed. 

What about not paying until the job is finished?

Make sure you keep that CT even if he refuses.. Get someone else to install 

You paid for it, he must give it to you, it came with the inverter.
That is probably the most important part for that type of inverter to work correctly.

What he is probably doing (which is what another installer tried to do with us in the past) is that he is using 'solar export' , which isn't allowed technically by the municipalities without registration, but if you have an old mechanical meter it will spin backwards. In that way your geyser etc will just be using the power you fed back into the grid. The problem is if they decide to change it for a digital meter, that plan won't work anymore, and that CT will be essential to feed solar to your non essentials

No need to research far and wide, just tell him to read the manual and do the job properly. Don't entertain any debates. Even if you're not installing solar panels right now, even if you're putting the whole house on the essential loads, just get it fitted, end of story because things may change in the future. The CT coil is key to getting full functionality out of your inverter. Something as seemingly small as the zero export constant power draw depends on it.