question

Hi @Chris White

53kw if you want to capture all the power across three phases assuming equally spread 53/3 = 17.7, as far as I'm aware the largest multiplus ii is 15kva. So you either use 3x 15kva multiplus ii and max out at 45kva across all three phase. (I would go this route unless it's critical for the extra load as this is far simpler)

Or you go the even more complex (and expensive) route of paralleling + three phase (I dont believe anything larger than 5kva multiplus supports parallel running so you'd have to use the 5kva model which would need ie 12 x multiplus ii of 5kva and do 4 in parallel and then in 3 phase. (I can't comment much on that as that is above my understanding).

Depending on your incoming grid connection this may be preferred/worth discussing/mentioning to DNO as I imagine they'll be quite involved in a system this large.

A few things to point out in your diagram is you've noted each single phase battery inverter to have it's own battery. In a Victron system the battery is the same pack/battery. You just have 3 different inverters if that makes sense. So essentially your diagram is correct but you have one battery with each of the three inverters connected to it rather than 3 individual batteries.

For a victron system you may also want to consider rewiring the above diagram so that the house is wired on AC OUT side of the inverters (ie not in parallel with the grid). This way you'll have power if the grid goes down. For systems from other companies you often see they have disconnects/ automatic transfer switches installed just after where your 3 phase grid meter is in the diagram however victron inverters have these built in so you can just wire the "critical loads" ie the stuff you want to stay on during grid outages "after" the victron inverters. (on ACOUT of the inverter rather than ACIN). You may see this refered to as Island mode from other providers and it often requires additional devices whereas it's built in on victron inverters.

You can also move the AC coupled solar to the ACOUT side if you prefer however you do need to consider the factor of 1 rule then. If you leave AC solar on grid side and house on critical load side you may want to consider adding some DC coupled PV just so you're not losing all solar if grid goes down.

ie Grid -> "battery inverter" -> house

Rather than in parallel like in your diagram.

Grid <-> battery inverter

Grid <-> house

In terms of battery sizing it really depends on your aims with the system. If it's to go purely offgrid then you'll need loads min 3 days worth of power at least and likely much more solar. However you may want to go a little smaller and take advantage of cheaper overnight electricity rates if you're not wanting full off grid. This is ideal for the winter rather than oversizing the solar to the extreme you charger batteries partially using cheaper electric and let solar keep you topped off during the day when it's darker in the winter.

If i scaled up my small house battery to 60kwh of inverters I'd say you'll want 250kwh (a days worth), at least. You'd need more than 50kw solar to be fully over grid in the winter I'd say.

Very rough approximation but In the winter I'd say 50kwh would be a good day on an array that size in the winter

I've found solar panels etc is cheaper than the batteries, so you're often better off financially if you up the solar rather than upping the battery as the battery tends to be the pricier part.

PS I'd absolutely love to offer to help complete work on a system this size but unfortunately I'm just a DIYer... Particularly the battery design, I've got designs for a 120kwh battery shelf that I'm dying to build.

@matt1309 Just thought that I would add that this is very much a Grid Tied system and we have only experienced one outage in the last 10 years that we can remember. We transform directly off the 10,000v overhead pylons to the premises. As I said our peak power usage during the day is about 40kw and the Solar inverters are able to cope with that so I don't think we would need more than about 10kw peak out of the batteries at any time. The only things running at night are A/C units on our Cellar which are fairly continuous loading. So we are looking at 10 hours of about 9kw. i.e. 90kWh total. It would be great if we could charge up the batteries at night if they empty or during the winter as we have a dual rate tariff that gives us cheaper electricity at night.