System with 2x 15Kva Multiplus’ and pylontech US5000B, what is the minimum battery qty required?
The more information you can provide will increase the usability of the cummunity’s responses and the quality of suggestions/recommendations.
If you are asking the THEORETICAL absolute minimum to feed a continuous 2 x 12kW peak power situations from the battery then the answer is 6 units(assuming 48V DC average voltage and 500A at peak power). 8 to be on the safe side. BUT, even then the C-rating of the current is about 0,6C. That is not healthy to the batteries when it comes to working lifetime of the battery units if said peaks are daily and long lasting(sub 0,5C is recommended). 0,3C or even lower would be preferred to get the max cycles of the batteries before 80% SOH. Personally I am using 0.1 to 0,2C(6000cycles estimated).
This is also why Pylontech is recommending the minimum for 15kVA units to be 5 x 100Ah units per inverter. 10 in total in this case.
Also there is the time window issue for charging nightly from the cheapest hours. How long is that window on average where the installation is gonna be located? Charging 400A with said 2 inverters would take 8 battery units(100Ah each) about 2 to 2,5h(again, with 0,5C current).
The optimum(if there is use and daily cycles) ROI for the whole system would be more like 2-3 times of said minimum amount of units(to provide better system level ROI and lower the C-rating).
My own residential setup is 3 x MP2-5000 in 3-phase setup using 60kWh of LiFePo4 batteries which is ideal for the size of the installation. So 120kWh(24 units) for your setup might not be overkill if there is use for the capacity(What kind of consumption are you planning on seeing on a daily average?).
Thou over 16 batteries brings added communication requirements: The Pylontech low-voltage combiner hub is required to combine more than 16 batteries, and allows the connection of up to 96 batteries.
All of this is of course speculation since we do not have enough information. This seems to be your first post, so we don’t know your background and knowledge of the matter, hence the long answer. Please provide more information to get more comprehensive suggestions.
EDIT. Just noticed the “Offgrid” tag of your post. What kind of Solar kWp and inverter peak is involved and is it MPPT as DC coupled or AC coupled like a Fronius or similar?
Thanks for your response, and all makes sense re battery outputs, my question was more around Victron generally specify minimum qty’s.in a table which you said is 5? but I can not find this info for the 15kVa Multiplus model. We will likely increase the capacity once the system is up and going and we have a bit more data on usage.
There will be a 450/200 mppt and a 10kw Fronius appprox 60-70 440W panels split across them.
Usage is slightly unknown with 3x heat pump run times dependant on usage (factored 3hrs each/day) and quite a few other loads that are seasonal, but expecting around high 30-40kwh / day. Idea is to program most of these for during the day with AC coupled to assist.
Solar array will be as large as possible to aid low psh during winter
Victron will not list every single combination of different model types from 3rd party battery manufacturers. They do provide a list of 100% tested compatible batteries for Victron’s BMS-Can communications. And like the link here, some inverter sizes and battery models for reference: Victron & Pylontech UP2500, US2000, US3000, US2000C, US3000C, US5000, US5000B, UF5000, Pelio-L, UP5000, Phantom-S, Force-L1 & L2 [Victron Energy]
After that, it is pure math and logic for each installer to calculate the power and energy needs of said installation.
In this case, the 15kVA Quattro(has both generator + grid support with 2 AC inputs) has the same power requirements as the same sized Multiplus. And the battery quantity is given in 50Ah smaller units. In your case you have 100Ah units, so if half of that. Hence 5 units. This is the recommended minimum for the power draw per inverter. Up to you to select the capacity.
10 would give 45,6kWh which seem quite suitable for the installation that you described.
I would go for 16 units if the budget allows and if the customer wants to have 2 day buffer for longer weak weather situations. Again, don’t know the situation, but the installation does not sound small and it is always better to get a bit oversized battery(especially off grid) than keep adding different ages of units that might behave differently. I have never heard that people regret on getting a bit larger battery than the minimum needed.
I also have the same exact Solar inverters of 450/200 MPPT via Lynx PowerIn and a 10kw Fronius connected to AC out1.
I over-paneled the 450 with 9 x 455W bifacial vertical x 4 giving me over 16kWp (11,2kWp for the Fronius install on the roof). Here in Finland the temps can go to -30C during winter, so not taking any chances to go over 450V and damage the PV charger. After this test winter time, I might add a 10th panel to each string if there is enough buffer with the max voltage at extreme cold days.
Sounds good, it was pretty well what I had figured but good to clarify- Thankyou very much for your help 
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