@BartChampagne Luckily I am in the fortunate situation of having 65kWh of battery capacity availabe and also an EV that can be charged when there’s no sun.
Each kWh can be sold to my employer for €0,45 (a lot cheaper than €0,69 for Fastnet).
So this “small” 10% efficiancy will pay off.
The two 250-100’s and one 150-35 cost me €1.280 a week ago.
Then the back-uo facility of having a stable 50Hz net during outages instead of a shifting frequency from 50-53 Hz (when batteres are full) when relying on the Fronius inverters listening to the net frequency of the inverting output…
Th third 250-100 (I already had one) can be connected to a 3kW DC supply (MPPT resistand shortcut safe) and a small generator to fill the batteries too.
If I would had to start all over again, I would make different choices but the road I entered is a familiar and trustworthy one so I am not complaining 
As long as you’re connected to the grid (also at zero consumption or feed in), your frequency will be a stable 50Hz, even when batteries are full.
The Fronius Zero Feed In feature will throttle down the output power so you’re not feeding to the grid, while staying at 50Hz grid frequency.
The 50-53Hz swing you’re talking about only counts for off-grid systems.
I’m not following you on why charging your EV at night from batteries will make a big difference in the choice of how those batteries were charged.
As stated: if you have excess solar (more then you can charge in the batteries) it doesn’t matter if the charge efficiency is a bit lower. Both the MPPT and the Fronius will throttle down when batteries are full
Only on days that you have more PV yield than you consume but not enough to charge you batteries to 100% you’ll benefit from using MPPTs.
It depends on your daily (daytime) power consumption and how much daily excess solar you can store in the batteries.
Personally I would keep at least one of your Fronius inverters for the daily AC consumption since that will have the highest efficiency for directly-consumed PV power and will take some inverting load off your Multi’s.
Double so if/when you’re (occasionally) charging the EV during daytime as that will also lower battery wear.
My own roof I’ve wallpapered with 88 panels of 450Wp.
16 of those are hooked to a 250/100 MPPT (might split to 2 MPPTs later) and the rest to 2x Fronius Symo 10kW as those were way cheaper than MPPTs.
So yeah, my own system is so very overdimensioned that, for me, the efficiency difference is not worth it.
And here in Belgium we can still get some € for injected solar
Each use case is different and I’m not trying to change your mind (since you already have the hardware), just sharing some best practices and my own use case.
@BartChampagne when having 88 panels on our roofs, I wouldn’t bother neither about some silly numbers of efficiancy 
I am aware that even 100 inverters can’t change the net frequency.
My point was when not connected toe the grid because of an outage.
Since we are running with 10kWp of panels, efficiancy counts imho.
With more batteries than pv produced energy I can charge my car when coming back home and the sun isn’t shining anymore. Even with 100 panels and no batteries you still have to pay.
And in our Dutchies country, they invented paying for feed-in, which is the last thing I want to do, hence the battery capacity.
Having that extra solarcharger, if one breaks, i can easily switch over to another or just split the whole system into 3 ones if needed.
Starting from scratch, I would go for a high voltage system in order to cover all cells in series with just one bms.
But I am happy with my decisions now.
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