Tnaik

It seems to be an inherent design, both my Axpert VM 3 and max does it in SUB. Although at low power draw say 500 watt the utility will supply about 400 w, however once runing at say 2kw, the utility supply will drop of to almost nothing. I have never seen the extreme  Thaink is refering to of 1kW from the utility if theres enought solar.

I guess I'm spoiled with my home made monitoring software. I assume that it must calculate the input power (it's been a while since I looked at that, and a colleague wrote most of it).
If you have an AC clamp meter, even a cheap one would do, you could clamp that over one (not both) of the AC-in wires.
Or just figure it out from other power figures, using an assumed efficiency of 97% (probably optimistic) and an idle load of 75 W (wild guess).

So how much power does it draw from utility when in SUB output source priority? If it's just a few hundred watts, that could be a buffer against pushing power towards the utility when loads suddenly decrease. 
Could it be the problem identified in other topics where under some conditions the MPPT algorithm gets stuck at a PV voltage well below the actual maximum power point? To help with this, can you report your PV voltage and power when it's using too much utility power, and then when you switch to SBU priority with a similar load? 
I've been quiet on this because you have an 8kW model, and there is no 8kW firmware to read. I do have several 7.2kW firmwares to read, but they likely won't work in your model. 
Also, is yours one of the newer models with the fixed display and the RGB LED ring around the display? These likely require different firmware to models with the removable display.
I have an idea for possibly fixing or working around the MPPT bug, but I can't patch 8kW firmware as I don't have a firmware update file for them. 

I can understand your frustration with SUB not working as it should. This mode is one of the great changes that was introduced for the MKS 111 and on the King and Max versions. It should actually use all the solar the panels produce and just add from the grid to make up the whole load. 
For your use you are correct that you need to ensure your batteries are kept full for the 8 hour a day power cuts. 
Perhaps you need the inverter checked why SUB is not working. I think some people got this mode working after updating software. 
In the mean time set the back to utility as high as you can. On my inverter it is setting 12.

You will have to decide what you really want. Perhaps setting 16 to SNU (solar and utility will charge at the same time) could satisfy your thoughts.

Hi Tnaik,
you can achieve this with setting 16 to CSO (charge source priority: solar first). No need to switch to SUB, remain in SBU.

Welcome you have a good system with a good size battery bank for a lead acid bank.
250ah x 48 volts= 12000watthours
50% of that should last you most of the night.
 
My suggestion would be to leave it on sbu but set the back to ac setting to what the battery manufacturer says corresponds with 50% dod.