question

Hi @justshowup

This is very doable. Victron systems are very versatile so you have several options.

You could make your whole house on UPS mode whereby you have 3 multiplus ii's or quatros that take the input of your grid and the grid tied solar and have your whole house off grid.

You can choose to wire grid tied solar "before" (AC IN) or "after" (AC OUT) of the three multiplus.

If you put them on AC OUT 1 then they'll stay on if grid goes down. The victron system is kind of making a mico grid in this scenario so solar will stay on if grid goes down. (You need to abide by factor of 1 rule in this scenario) and enable frequency shifting (I already know Fronius supports this feature) to ensure Victron system can turn off fronius/ramp it down in the even there's not enough load to use the solar (ie batteries are full and grid is off line).

This is more expensive as it's a whole house setup as you need 3 inverters for three phase setup, and you also need them large enough to pass through all of your grid power if you dont want to compromise on that (however 3xMultiplus 5kva sounds amble). The other option is to have just one multiplus in parallel with grid. Assuming all critical equipment is single phase.

ie rather than

Grid -> multiplus -> whole house

You instead have

Grid -> SUpported energy meter -> whole house

&

Grid -> multiplus -> critical loads only. (this would be cheaper as you could just do this on a single phase ie one multiplus)

The multiplus in this configuration can still "back feed" to cover house loads if you have a victron supported meter installed so that the system can calculate how much to back feed in the event the batteries are full/if you have DC coupled solar. However it cant back feed if grid goes down as that will send power out to grid network potentially shocking someone. However the critical loads can stay on (in the earlier example the whole house would be on critical loads side).

In this example you'd have to also have solar in parallel with the grid as you'd only have one multiplus (you'd need three to make a micro grid to have AC coupled PV (fronius) stay online if grid goes down).

One this to note is I'm unsure on the speed of transfer switches for victron (ie it may not compete with purpose built UPS) so you may want a tiny UPS to just help with that switching speed if devices are critical to have 100% up time.

I've barely scratched the surface with the above. However I've personally found if you think "wouldnt it be good if a solar setup could work like xyz" odds are victron supports xyz in some form or another.

Here's some useful links to understand the full features of victron systems:

1. ESS Introduction & features (victronenergy.com)

AC-coupling and the Factor 1.0 rule [Victron Energy]

If price is no object, then by all means, put your entire house on the output of the inverter. You'll need a lot of batteries for this however to accommodate the 10 KVA PV inverter on the output. That is covered in the same one-to-one rule discussion.

20 mS is one cycle of the 50 Hz AC power. Most devices won't be affected by this short interruption. "Off-line" UPS have the same switchover delay since they need to detect loss of AC and then switch between passing incoming AC to the inverter output.

Another issue with using Multi or Quattro as a UPS: when the AC input is in use, the voltage and frequency are determined by the incoming grid. If the incoming voltage drops, the output will also drop. Some off-line UPS have a "boots/buck" capability to compensate for incoming line variations. Multi/Quattro won'd do this! You can set the voltage thresholds for the incoming AC to minimize brown-outs but this means the load would be supplied by the batteries until the incoming AC returns to an acceptable value. So one or more separate UPS do have a slight advantage.

If 20 mS interruptions are not acceptable you need an "on-line" UPS. In this type of UPS, the inverter runs all the time feeding the AC output from the battery. A separate charger keeps the battery charged when incoming AC is available. There is zero interruption in AC output. An on-line UPS also completely eliminates brown-out problems.

@justshowup Just my 2cts

I did exactly what you want to do except I only have 1 phase. Behind the Multiplus I have a few Synology NAS with Docker and VM's. A few Ubiquity AP's, FritzBoxes, Raspberry Pi's etc. I have had many (self introduced) power failures. Not brownouts but real blackouts. Never one of the systems behind the MP had any problem and continued to work fine. 20mS is a short time for a AC-DC adapter but 1 AC cycle are still 2 DC cycles after the bridge rectifier. :-)

That said, most, at least a lot of equipment works on 12VDC (or lower). Therefore I'm going to power most critical devices via an Orion DC-DC converter directly from the battery. Its more efficient too but the devices must be relative close to the MP of course.

--Gerard

Thank you for your input @Kevin Windrem!

Just so I understand everything: to eliminate all kind of power outage 100%, I would need an "on-line" UPS, right? So theoretically if I could find an "on-line" 3-phase 230V UPS with big enough battery I could just plug that in (where exactly would this go?) and it's all good? In this case though, my solar panels wouldn't provide anything in a black out, since the inverter would be shut off, right?

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Thanks @Gerard van Seventer, that helps! :) "Sadly" it's a big house and the stuff are everywhere, that's why I'm looking for a "global" solution, if possible, it just feels a cleaner.

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So in theory the ideal/best scenario would be to have a MultiPlus 15 kVA inverter (I'm assuming it's 3-phase), the BYD batteries connected to the MultiPlus inverter, the solar panels connected to the Fronius inverter which itself is connected to the MultiPlus, right? In this case during black out I could still use solar power during the day, just wouldn't feed back to the grid.

Regarding the "on-line" UPS: where would that go in this ideal setup? I just can't place it. Also, how "powerful" does that "on-line" UPS would need to be in my case? Does it have to have a big battery included or is it simply there to handle those switches "instantly", but other than that it doesn't matter? Can any of you recommend a UPS for that?

Edit: if there is an online UPS anyways, wouldn't it make sense to just have one with a big battery and just completely ditch the BYD batteries?

Also, it feels like (but please correct me if I'm wrong) that the Fronius inverter is basically redundant in this whole setup and the Victron MultiPlus would be able to handle everything. Is there any reason to keep the Fronius in place?

Regarding an on-lne UPS:

There are two ratings you need to consider: peak power (number of watts of all your equipment); and run time at that load. Run time would only be a factor if you put the UPS on the AC in side of the Multis since it would then need to power loads when the grid is down. So the best place is on the output of the Multis.

Large on-line UPS can be expensive so it might make more sense for smaller ones at each cluster of loads. One thing to consider: an on-line UPS will have a fan running all the time. APC units tend to run the fan at full all the time so might want to look for one with variable speed fan. Or plan on locating it out of living/working spaces.

I highly doubt you'd need an on-line UPS (or any with the Mutis powering the entire house) but again if price is no object, why not.

I have 4 1000-1500 VA off-line type UPS units in my house and none of the devices they power have a problem with a switchover. (I don't currently have an inverter system for the house, only a small generator. If/when I do what you are considering, the UPS units will be taken out of service.)

I also run my main router, LTE modem, main ethernet switch, my load management system and VOIP modem running off of a 12 volt lithium battery. It has enough storage to run everything through the night when the generator is shut down.

You could consider a central UPS but then wire the house for separate UPS outlets for those super-critical loads. The UPS would then feed a small breaker panel that feeds the UPS outlets.

The batteries in the UPS will require replacing about every 3 years. There are some high end UPS units that use LFP batteries so they would need less in the way of maintenance. As an example the N1 Critical Technologies N1C 3000 VA on-line UPS is on Amazon in the US for $2,800.

Note that UPS units are generally single phase.

I would not consider the Fronius redundant. Assuming you have enough solar, you can size your battery bank to run for just time without sun plus battery recharge time. Without it, you'd probably want to size your battery bank for a couple of days, or at least one full 24 hour period. You may also need to incorporate a generator. It's also "free" energy, minimizing the power you pay for from the grid. So keep the solar.