question

Couple of things worth mentioning:

1) You should start by finding out the model of your compressor by taking a peek on he back of your fridge. With that you should be able to find its datasheet and in there there is a spec typically called "cut in current" stating the current required at startup. By multiplying that times your voltage you can figure out how much is the peak in Watts and determine whether your inverter is properly dimensioned. This for example is my compressor's datasheet: https://www.secop.com/fileadmin/user_upload/SEPS/datasheets/hxd30aa_cdo00097_r600a_220v_50hz_11-2016_desd563i102.pdf

2) With the above being said I would distinguish between the error messages you get. The "overload alarm" message is a clear statement meaning your inverter is not powerful enough (most of the time just because of the in rush current required). If you didn't get this error I don't think the 12/800 model will necessarily help the ECO-mode issue.

Messages like "low battery voltage" just imply you battery is not big enough to handle the current peak required on the 12V side during the initial inrush current phase.

The few information related to the "High AC voltage" alarm I found are not entirely clear to me, mentioning something about the wires used on the AC side. But still in my case I do not understand the real reason for this alarm.

3) I have been monitoring the consumption of my fridge for quite a bit and I did notice that even if tipically the consumption cycle is ~30W / 0W when ON/OFF somwtimes I saw it jumping up to 200W for extented period of time (up to 5min or so). Even if I am not entirely sure the reason for this I can tell that this is not related to the ECO mode pulses. Just normal behavior when the fridge is connected to the regular home 230V outlet. This to me means that for whatever reasons sometimes the compressor finds itself in a state in which it draws way more current then normal. My feeling this is the same state you are referring to when you say "the compressor stalls". If that is the case I would consider this state still normal behavior since as mentioned I saw it happening more than once on a new fridge running on 230V (no inverter). With that being said it is entirely possible that this is caused by the motor stopping in a specific position that for some reason is not favorable to restart, but at least in my case it eventually recovers.

4) I don't know about your case, but in my case the off grid solar system is installed up in a hut in the mountains which can only be reached after 45min of steep uphill hike. For this reason I need to thrust my inverter, which means I have no problem spending a couple hundred dollars more for a quality inverter. In the case of Victron Energy products I have been very positively impressed with their quality, so personally I would think twice before going to a cheap/no-brand inverter.

5) Last but not least, the ECO-mode is not the only highlight of the Phoenix inverter. It's low zero-load consumption is also pretty good. So in your case, if we are talking about a 40W fridge you should just add 5-6W of the inverters consumption which in terms of percentage means an increase of less than 15%.

6) One more.... the point I was trying to make is that in my view the problem we are experiencing in ECO mode is more related to the pulse width. This appears to be confirmed by the fact that everytime I would get the High voltage alarm by switching to OFF and then back ON it would always recover (I was doing this from the phone app, not even from the physical switch). And BTW, I personally perfomed all my test with the interval set to 60s, because in a fridge I do not think a 3s poll or so is required. In addition my gut feeling is that a compressor might not enjoy this pulses of current slaps overtime.

This are really just my 2 cents.

Thank you very much for your inputs, it is very helpful to share experiences with other people with the same issue. I would be nice though having someone from VE step in as well.

Thanks a lot

The 200w could be a defrost or oil-heating load.

Very short polling-time may give some crank's which build up pressure. Higher pressure higher inrush, so poll every few minutes (3-5min or longer) and for 2-3seconds long. Just count out the seconds via ear to estimate the fridges start-up time and add a second.