MGN

Your AC voltage to the fan is probably changing.
When there's grid, even if running off battery, the inverter matches grid voltage and frequency.
When there's no grid, the inverter will output the 'Inverter Output' voltage specified in the settings instead; usually this will be set to 230V in SA.
If you check your stats, you'll probably find that grid voltage is significantly different (probably lower) than the 230V the inverter supplies when running without grid.
At my house, the supposed CoJ supply (which should always be around 230V) often drops all the way down to 195V (!). Check yours and you'll probably see something similar (although hopefully not as extreme as my case!)
At differing voltages, your fan (and some other electronics: my dimmable lights change brightness, for example) will behave a bit differently.
 
 

Suggestion would be to investigate appropriate grid parameter limits (Voltage and frequency) for the inverter's settings, so the inverter only transfers from battery to grid once the grid has stabilised after re-connection. The re-connection time is usually 60 seconds, which should be enough time to track the phase of the grid.
Just my 2c.

The axpert employs grid tieing internally in the same manner the Sunsynk does, with the only exception being that the Axpert is programmed not to allow exporting of grid power. This incidentially is the reason why Axpert can also blend grid with the inverter output. 
The Axpert will synchronise to grid and when done, the input grid safety relay is energized and this connect grid, inverter and load together (given the load relay is also energized). 
The only barrier between the inverter output and the grid input is a reactance in the form of  large inductor. This is the grid tie. The direction and magnitude of current flow between grid and inverter is now controlled solely by the relation of voltage phase angle (not voltage magnitude) between grid and inverter. When the grid is charging the battery or blending the load, this phase angle is adjusted accordingly.  
So the notion of relay switching to accomplish changeover from grid to inverter does not exist anymore. It is now seamless. 
However, when grid falls away when it was carrying substantially the load, there will be metastable conditions in order for the inverter to instantsneously absorb and cope with the full amount of load current. I suspect this is where some flickering is visually observed.  However this condition does not affect my computer workstation and other appliances which is dependant on non-interruption. A fridge will certainly not be affected by this. 

I get that too with my Goodwe. Round where I live the voltage seldom gets to 220, so when load shedding starts the voltage switches from whatever was on the grid to the nice 230 the Goodwe puts out. And there's a small but finite period of time to change over. So a slight flicker of the lights seems to me to be expected. 
I also get a flicker on restore. This is the reverse situation with the Goodwe having to drop down from 230 to grid. 

I have no experience with Hubble. 
Freedom Won... I used to have a different brand of battery, and there were many little glitches with the system. 
Then I switched to Freedom Won. 
Shortly after the whatsapp chat between myself & the installer pretty much dries up. I tell him "that's much better", this is round about the time Eskom started piling on the load shedding, and then it all goes quiet. 
So in my experience it's a quality product. My system has (bar one instance of finger problems) been rock solid since the Freedom Won was installed, though it's not the model you're considering. 
Hubble & Freedom Won can both be charged & discharged at a faster rate than the Pylontech. That feature might come in handy at current levels of loadshedding and will allow you to run bigger loads. 

Blue nova 12v 8ah lithium battery should do easily do the job. 
 
Blue nova 8ah

HI All,
I'm not sure if this is the right place to ask, but I've read some really useful forum threads here about rainwater harvesting, so here's my question:
I've got a rainwater harvesting tank and small pressure-controlled pump which is currently totally independent from my municipal supply. However I'd like to put a couple of toilets and washing machine onto rainwater, but obviously can't guarantee a year-round supply. I should be able to isolate those 3 supply points onto one line such that they can be fed by rainwater, and then switched over to municipal if I don't have any rainwater in the tank. The switch would just be at one location - I don't want to have to drill new holes into the water at all 3 places and have some kind of switch (eg: below the toilet) to manually switch over each one - that would be madness. 
What this means though, is that these fittings (2 toilets and washing machine) would be supplied via their existing copper piping downstream of the "switch over" point. I've heard a few people say that you should not use copper pipe with rainwater as it tends to be acidic and may corrode the pipe. I'll wont be drinking this water, but I don't want to have to replace the pipes after a couple of years. Does anybody have any experience with this/know much more about it? Is just a bad idea if you plan to drink the water, or will the acidity of the rainwater destroy the pipes pretty quickly?
Thanks in advance
Nick

The System I use do start the pump when the collector reaches 5 degrees . This stops the collector from freezing . The Geyserwise controller is used .